After the death of General Władysław Sikorski in July 1943, Sosnkowski became Commander-in-chief of the Polish Armed Forces.

Sosnkowski was an intellectual who was able to speak Latin, Greek, English, French, German, Italian, and Russian.

His father, Józef Sosnkowski of the Godziemba coat of arms, was a wealthy nobleman and owner of several villages.

[1] Already in 1904, having met Józef Piłsudski, the future leader of Poland, and influenced by him, Sosnkowski joined the Polish Socialist Party (PPS).

In 1910, the union created paramilitary units the Riflemen's Association (Związek Strzelecki) (in Lwów) and Strzelec (in Kraków).

Sosnkowski conducted an energetic counter-offensive and recovered most of the lost ground,[2] even though he was considered to be primarily an army organizer, with limited field experience.

[1] In 1922, Marshal Piłsudski sent his confidential opinion to the president of Poland in which he declared that only Sosnkowski and Edward Rydz-Śmigły are capable of being commander-in-chief in case of war.

After resigning from his ministerial position, Sosnkowski returned to active duty as member of the War Council and commander of the VII Corp District.

In 1925, as the Polish permanent representative to the League of Nations, Sosnkowski initiated the adoption of the first international instrument addressing biological weapons of mass destruction: the Geneva Protocol for the Prohibition of Poisonous Gases and Bacteriological Methods of Warfare.

When President Stanisław Wojciechowski demanded during the coup that military forces come to the government's aid, Sosnkowski dispatched the units under his command accordingly.

He became head of the Committee for Matters of Armaments and Equipment and remained in that position until the outbreak of World War II.

[1] On 16 August 1920, Sosnkowski ordered the internment of Jewish soldiers, officers, and volunteers of the Polish Army at a camp in Jabłonna, 14 miles north of Warsaw.

The order made reference to "the continuous increase in cases testifying to the harmful activities of the Jewish element", which supposedly validated their alleged pro-Bolshevik sympathies.

[4] Some Poles protested, including Prime Minister Ignacy Daszyński, who called the order shameful and demanded the Polish-Jewish inmates’ immediate release and return to active duty.

Jewish members of Polish parliament expressed outrage, writing to Sosnkowski on 19 August 1920 that "such orders instill the conviction that Jews are enemies of the state…".

[4] At a parliamentary session held on 29 October 1920, the Zionist deputy Yitzhak Gruenbaum demanded an explanation from Sosnkowski.

"Jabłonna was ordered," Sosnkowski replied, "at a time when the enemy was at the gates of our capital, when Praga (the east bank district of Warsaw) was aflame."

Sosnkowski stated that reports of Polish-Jewish soldiers laying down their arms and joining the Bolsheviks forced his hands.

Gruenbaum interjected, asking Sosnkowski to provide the name of a single Jewish soldier who was reported to have committed such an act of treason.

His ideas were rejected by Rydz-Śmigły, which may have resulted in uncoordinated military activity and eventually the Polish defeat in the Battle of the Bzura.

For the duration of the government's stay in France he was also commander of the Union of Armed Struggle (ZWZ), which was established on 13 November and operated in Poland.

Sosnkowski resigned from the Polish government because of the Sikorski–Mayski agreement of 30 July 1941 and protested the lack of specifics regarding Poland's future eastern borders.