After the outbreak of the Great War in August 1914, Stachiewicz joined the Polish Legions in which he became a platoon commander in the 1st Regiment.

On 9 October, he was promoted to second lieutenant and sent with a secret mission to the other side of the Russo-Austrian Front to help the creation of Polish underground organisations in the territory that was still under Russian occupation.

However, the Oath Crisis of 1917 caused Stachiewicz to be drafted into the Austro-Hungarian Army, demoted to sergeant and sent to the Italian Front.

After the death of Marshal of Poland, Józef Piłsudski, Stachiewicz's place was taken by General Edward Rydz-Śmigły, who nominated him for the post of the Chief of Staff of the Polish Army.

After the Soviets joined the war on the side of the Nazis, he crossed the border on 18 September with Romania to continue the struggle abroad in France.

In January 1940, Stachiewicz managed to escape from captivity and, through Bucharest and Yugoslavia, reached the French-held port of Algiers.

However, General Władysław Sikorski insisted for another internment, this time by the French, and it was not until 1943 that Stachiewicz finally reached London.

After World War II, he was deprived of his Polish citizenship by the Soviet-backed communist authorities of Poland and had to remain in exile.