Upon the outbreak of World War I in 1914, he became chief of the military department in the Polish National Committee (Polski Komitet Narodowy) and remained in this post until 1916. Later, as a commissioner of the Polish Legions in Kraków, he was responsible for recruitment to the Legions, an army created by Józef Piłsudski to liberate Poland from Russian and, ultimately, Austro-Hungarian and German rule. The Legions initially fought in alliance with Austro-Hungary against Russia. From 1916 there was growing tension between Sikorski, who advocated for cooperation with Austro-Hungary, and Piłsudski, who felt that Austro-Hungary and Germany had betrayed the trust of the Polish people. In June 1917 Piłsudski refused Austro-Hungarian orders to swear loyalty to the Austro-Hungarian emperor (the "oath crisis," kryzys przysięgowy) and was interned at the fortress of Magdeburg, while Sikorski returned to the Austro-Hungarian Army. Although in 1918 Sikorski came to agree with Piłsudski (and soon joined Piłsudski in internment), from now on the two great Polish leaders would drift farther and farther apart.

Władysław Sikorski - Polish-Soviet War

In 1918 the Russian, Austro-Hungarian and German empires collapsed, and Poland once again became independent, but the borders of the Second Polish Republic were not stable. On the east they would be determined in escalating conflicts among Polish, Ukrainian, Baltic and Soviet forces in what culminated in the Polish-Soviet War of 1919 to 1921. In the opening phase of the war, Władysław Sikorski, now commander of the Polish Army in the Galicia region, took part in the liberation of Lwów and Przemyśl. Later Sikorski commanded the Polesie Group during Poland's Kiev offensive in early 1920. He had a good working relation with French General Maxime Weygand of the Interallied Mission to Poland.

In April 1920 the Red Army of Russia's new Soviet regime pushed back the Polish forces and invaded Poland. Subsequently Sikorski failed to hold the Brest fortress, but then distinguished himself commanding the Polish 5th Army (the Lower Vistula front) during the Battle of Warsaw, when Soviet forces, expecting an easy final victory, were surprised and crippled by the Polish counter-attack. During that battle (sometimes referred to as "the Miracle at the Vistula") Sikorski stopped the Bolshevik advance north of Warsaw and gave Józef Piłsudski the time he needed for his counter-offensive; for his valorous achievements Sikorski received the highest Polish military decoration, the order of Virtuti Militari. After the Battle of Warsaw, Sikorski commanded the 3rd Army during the latter stages of the Battle of Lwów and the Battle of Zamość, and then advanced with his forces toward Latvia and deep into Belarus. The Poles defeated the Soviets, and the Polish-Soviet Treaty of Riga (March 1921) gave Poland substantial areas of Belarus and Ukraine. Sikorski's fame was vastly enhanced as he became known to the Polish public as one of the heroes of the Polish-Soviet War. He would describe his role in the war in a 1923 book, Nad Wisłą i Wkrą (At the Wisła and Wkra [Rivers]).

Władysław Sikorski - In government and in opposition

In April 1921 Sikorski succeeded Piłsudski as commander-in-chief of the Polish Armed Forces, and became chief of the Polish General Staff. Between 1922 and 1925 he held high government offices. After the assassination of Polish President Gabriel Narutowicz, the Marshal of the Sejm (the Polish parliament), Maciej Rataj, appointed Sikorski prime minister. From December 18, 1922, to May 26, 1923, Sikorski served as Prime Minister and also as Minister of Internal Affairs. During his brief tenure as prime minister, he became popular with the Polish public and carried out essential reforms in addition to guiding the country's foreign policy in a direction that gained the approval and cooperation of the League of Nations. He also obtained recognition for Poland's eastern frontiers from Britain, France and the United States. From 1923 to 1924 he held the post of Chief Inspector of the Armed Forces. From February 1924 to 1925, under Prime Minister Władysław Grabski, he was Minister of Military Affairs and guided the modernization of the Polish military. His proposal, however, to increase the powers of the Minister of Military Affairs while reducing those of the Chief Inspector of the Armed Forces met with sharp disapproval from Piłsudski. From 1925 to 1928 Sikorski commanded Military Corps District (Okręg Korpusu) VI in Lwów.

A democrat and supporter of the Sejm, Sikorski maintained his neutrality during Józef Piłsudski's May coup d'état in 1926, which was supported by most of the military. In due course, as a semi-dictatorial Sanacja regime was established, Sikorski joined the anti-Piłsudski opposition. In 1928 he was dismissed by Piłsudski from public service and transferred into the reserves. In 1936, together with several prominent Polish politicians (Wincenty Witos, Ignacy Paderewski, and General Józef Haller) he joined the Front Morges, an anti-Sanacja political grouping. Sikorski largely withdrew from politics, spending much of his time in Paris, France, and working with the French École Superieure de Guerre (war college). Based on his experiences in the Polish-Soviet War, he wrote a book on the future of maneuver warfare, Przyszła wojna – jej możliwości i charakter oraz związane z nimi zagadnienia obrony kraju (War in the Future: Its Possibilities and Character and Associated Questions of National Defense, Polish and French editions 1934, English edition 1943), advancing ideas similar to the German concept of Blitzkrieg ("lightning war"). Alongside France's Charles de Gaulle and Russia's Mikhail Tukhachevski, he may be considered one of the pioneers of Blitzkrieg theory. During this period, he wrote several other books and many articles, foreseeing, among other things, the rapid militarization of Germany and the deleterious effects of Western appeasement policies.

As the international situation deteriorated, Sikorski returned to Poland in 1938, hoping to be of more active service to his country.

Władysław Sikorski - Prime Minister in Exile

When Poland was invaded by Germany in September 1939, Sikorski was refused a military command by the Polish Commander in Chief, Marshal Edward Rydz-Śmigły. Sikorski escaped to Paris, where on September 28 he joined Władysław Raczkiewicz and Stanisław Mikołajczyk in a Polish government-in-exile, becoming from September 30 the most successful, credible, and famous of the Polish prime ministers in exile. He preserved the continuity of his country’s government and was respected and recognized by the population of occupied Poland. During his years as prime minister in exile, Sikorski personified the hopes and dreams of millions of Poles, as reflected in the saying, "When the sun is higher, Sikorski is nearer" (Polish: "Gdy słoneczko wyżej, to Sikorski bliżej"). On November 7 he became Commander in Chief and General Inspector of the Armed Forces (Naczelny Wódz i Generalny Inspektor Sił Zbrojnych).

His government was recognized by the western Allies, as Poland, even with its territories occupied, still commanded substantial armed forces: the Polish Navy had sailed to Britain, and many thousands of Polish troops had escaped via Romania and Hungary or across the Baltic Sea. Those routes would be used until the end of the war by both interned soldiers and volunteers from Poland, who jocularly called themselves "Sikorski's tourists" and embarked on their dangerous journeys, braving death or imprisonment in concentration camps if caught by the Germans or their allies. With the steady flow of recruits, the new Polish Army was soon reassembled in France and in French-mandated Syria. In 1940 the Polish Highland Brigade took part in the Battle of Narvik (Norway), and two Polish divisions participated in the defense of France, while a Polish motorized brigade and two infantry divisions were in process of forming. A Polish Independent Carpathian Brigade was created in French-mandated Syria, to which many Polish troops had escaped from Romania. The Polish Air Force in France comprised 86 aircraft in four squadrons. One and a half of the squadrons were fully operational, while the rest were in various stages of training. At that time Poland was the third most powerful Ally, with some 84,000 soldiers in France alone.

Although many Polish personnel had died in the fighting or had been interned in Switzerland following the fall of France, General Sikorski refused French Marshal Philippe Pétains proposal of capitulation to Germany. On June 19, 1940, Sikorski met with British Prime Minister Winston Churchill and promised that Polish forces would fight alongside the British until final victory. Sikorski and his government moved to London and were able to evacuate many Polish troops to Britain. After the signing of a Polish-British Military Agreement on August 5, 1940, they proceeded to build up and train the Polish Armed Forces. Experienced Polish pilots took part in the Battle of Britain, where the Polish 303 Fighter Squadron achieved the highest number of kills of any Allied squadron. After the creation of the pro-German Vichy government in France and the ensuing split of French forces, the Polish Army in the United Kingdom and the Middle East became the second largest Allied army after that of the United Kingdom.

Following the German invasion of the Soviet Union ("Operation Barbarossa") in June 1941, General Sikorski was among the first to realize that the complexion of the war had drastically changed. Strongly encouraged by British Foreign Office diplomat Anthony Eden, Sikorski on July 30, 1941, opened negotiations with the Soviet ambassador to London, Ivan Maisky, to re-establish diplomatic relations between Poland and the Soviet Union, which had been broken off after the Soviet invasion of Poland in September 1939. Later that year, Sikorski went to Moscow with a diplomatic mission (including the future Polish ambassador to Moscow, Stanisław Kot, and chief of the Polish Military Mission in the Soviet Union, General Zygmunt Szyszko-Bohusz). Sikorski was the architect of the agreement reached by the Polish Government with the Soviet Union (the Sikorski-Maisky Pact of August 17, 1941), confirmed by Joseph Stalin in December of that year. Stalin agreed to invalidate the September 1939 Soviet-German partition of Poland, declare the Russo-German Molotov-Ribbentrop Pact of August 1939 null and void, and release tens of thousands of Polish prisoners-of-war held in Soviet camps. Pursuant to an agreement between the Polish government-in-exile and Stalin, the Soviets granted "amnesty" to many Polish citizens, from whom a 75,000-strong army (the Polish II Corps) was formed under General Władysław Anders and evacuated to the Middle East, where Britain faced a dire shortage of military forces. The whereabouts of thousands more Polish officers, however, would remain unknown for two more years, and this would weigh heavily on both Polish-Soviet relations and on Sikorski's fate.

Nonetheless, it soon became clear to Sikorski that the Soviet Union still had post-war plans for Polish territories, involving concessions to which he had no mandate from his nation to accede. The Soviets began their diplomatic offensive after their first major military victory in the Battle of Moscow. In January 1942 the Soviets through diplomatic channels revealed their claims to the city of Lvov. On January 26 British diplomat Stafford Cripps informed General Sikorski that, from what he had privately learned in Moscow, Stalin planned to annex Germany’s East Prussia to Poland in the west, but also to considerably push westward Poland’s eastern frontier, along the lines of the Versailles concept of the Curzon Line. Sikorski commented: "In short, to push Poland over from east to west.[...] But that cannot be done without Polish consent." Sikorski insisted that there could be no question of Poland emerging from the war with territorial losses: "The principles of the Atlantic Charter and the terms of the Treaty of Riga alone may determine the eastern frontiers of Poland." His unyielding stance soon proved to be an increasing hindrance not only in Polish-Soviet relations, but also for the British-American-Soviet alliance. British Prime Minister Winston Churchill and United States President Franklin D. Roosevelt were increasingly torn among their commitments to their Polish ally, the uncompromising stance of Sikorski, and the demands — often verging on political extortion — by Stalin and his diplomats. Soviet intentions were made clear in a comment by Ambassador Ivan Maisky to Churchill, that Poland's fate was sealed as "a country of 20 millions next door to a country of 200 millions."

Władysław Sikorski - Katyn

In 1943 the fragile relations between the Soviet Union and the Polish government-in-exile finally reached their breaking point when, on April 13, the Germans announced the discovery of the bodies of 4,000 Polish officers who had been murdered by the Soviets and buried in Katyn Forest, near Smolensk, Russia. Stalin claimed that the atrocity had been carried out by the Germans, while Nazi propaganda orchestrated by Joseph Goebbels successfully exploited the Katyń massacre to drive a wedge between Poland, the Western Allies and the Soviet Union. The Soviet Union/Russia did not acknowledge its responsibility for this and similar massacres of Polish officers until the 1990s.

When Sikorski refused to accept the Soviet explanation and requested an investigation by the International Red Cross on April 16, the Soviets accused the government-in-exile of cooperating with Nazi Germany and broke off diplomatic relations on April 26. With the action, the intentions of the Soviet Union had at last become clear: it wanted the Baltic States, which Poland had always considered as lying within her own sphere of interest; and moreoever it wanted to establish the Curzon Line border, which the government-in-exile (and least of all General Sikorski) would never accept, as it would mean the loss of about a third of Poland's territory.

The Soviets were able to sever relations with the Polish government-in-exile by exploiting controversy over an atrocity which they had themselves committed against Polish forces. They were also able to clear the way for a postwar communist-sponsored Polish government (PKWN) which would yield compliantly to Soviet demands. Stalin soon began a campaign for recognition by the Western Allies of a Soviet-backed Polish government led by Wanda Wasilewska, a dedicated communist with a seat in the Supreme Soviet, with General Zygmunt Berling, commander of the 1st Polish Army in Russia, as commander-in-chief of all Polish armed forces.

Władysław Sikorski - Death

On July 4, 1943, while Sikorski was returning from an inspection of Polish forces deployed in the Middle East, he was killed when his plane, B-24 Liberator AL523, crashed into the sea 16 seconds after takeoff from Gibraltar at 23:07 hours. He was buried in Newark-on-Trent, near Nottingham, England. On September 17, 1993, his remains were transferred to the royal crypts at Wawel Castle in Kraków, Poland.