After the German invasion of 1939 he proposed forming a group of armies in the regions of Warsaw, Poznań, Pomerania and Łódź.
This was rejected by Rydz-Śmigły due to the lack of coordination in the region of Warsaw and Kutno, and eventually resulted in the Polish defeat at the river Bzura.
On 10 September 1939 he was appointed commander of the group of the southern armies and conducted several victorious battles, however the Soviet invasion of 17 September made it impossible to continue the war effort and Sosnkowski, disguised, crossed the Soviet occupied territory of Poland and through Hungary.
He arrived in France in October 1939.
In November 1939 he was selected by President-in-exile Władysław Raczkiewicz as his successor (against the wishes of General Władysław Sikorski).
He was also appointed the Commander of the Union of Armed Struggle (ZWZ).
In 1941, he resigned from the Polish Government in Exile protesting lack of specifics regarding Polish eastern borders.
After the tragic death of General Sikorski in July 1943, Sosnkowski officially became the Commander-in-Chief.

 He was against starting an insurrection in Warsaw but Prime Minister Stanisław Mikołajczyk prevailed and the insurrection started in August 1944.
Sosnkowski then turned to the allies for help.
When his propositions were rejected, Sosnkowski officially accused the Allies of breaking the peace treaties.
Under the pressure from Winston Churchill Sosnkowski was demoted from the Commander-in-Chief position on 30 September 1944.
In November 1944 he left Great Britain and went to Canada.
Because of his unyielding attitude toward the Soviets, he was denied US and British visas until 1949.
After the German invasion of 1939 he proposed forming a group of armies in the regions of Warsaw, Poznań, Pomerania and Łódź. This was rejected by Rydz-Śmigły due to the lack of coordination in the region of Warsaw and Kutno, and eventually resulted in the Polish defeat at the river Bzura. On 10 September 1939 he was appointed commander of the group of the southern armies and conducted several victorious battles, however the Soviet invasion of 17 September made it impossible to continue the war effort and Sosnkowski, disguised, crossed the Soviet occupied territory of Poland and through Hungary. He arrived in France in October 1939. In November 1939 he was selected by President-in-exile Władysław Raczkiewicz as his successor (against the wishes of General Władysław Sikorski). He was also appointed the Commander of the Union of Armed Struggle (ZWZ). In 1941, he resigned from the Polish Government in Exile protesting lack of specifics regarding Polish eastern borders. After the tragic death of General Sikorski in July 1943, Sosnkowski officially became the Commander-in-Chief.
 He was against starting an insurrection in Warsaw but Prime Minister Stanisław Mikołajczyk prevailed and the insurrection started in August 1944. Sosnkowski then turned to the allies for help. When his propositions were rejected, Sosnkowski officially accused the Allies of breaking the peace treaties. Under the pressure from Winston Churchill Sosnkowski was demoted from the Commander-in-Chief position on 30 September 1944. In November 1944 he left Great Britain and went to Canada. Because of his unyielding attitude toward the Soviets, he was denied US and British visas until 1949.
Kazimierz Sosnkowski