After the outbreak of the Great War he joined Józef Piłsudski's Polish Legions, where he served in the 1st Uhlans Regiment.

When Poland regained her independence in 1918, Kozłowski volunteered for the Polish Army and served with distinction during the Polish-Soviet War.

After death of Marshal of Poland Józef Piłsudski, Kozłowski remained an active politician for some time.

However, after the Sikorski–Mayski agreement of 1941 he was released and travelled to Buzuluk, where he tried joined up with the Polish Army formed there by general Władysław Anders.

He then left the military camp and started his 1000-mile long trip westwards, accompanied by one officer, intending to join the Germans.

Though he had expressed doubts of the version of the events as presented by the Reich Ministry for Propaganda under Joseph Goebbels, he became convinced of the Soviet guilt after having seen the mass graves and conversed with the members of the expert commission of the Polish Red Cross under forensic doctor Marian Wodziński[1] The pro-German Polish language daily Goniec Krakowski cited Kozłowski as saying that he had recognized acquaintances among the dead bodies, among them professors from Lwów and ministerial officials from Warsaw.

[2] Interned in Berlin (at the Hotel Alemannia), Leon Kozłowski found work at an ethnographic museum and could pursue scientific research.