During the German attack on Poland in 1939, he bravely commanded troops around Warsaw. After a brief imprisonment, he escaped to France, where he began training Polish soldiers.
Following the fall of France, he continued this duty in Scotland until 1944.

In the British Isles, his efforts led to the creation of the 1st Independent Parachute Brigade. Sosabowski hoped his elite unit would be deployed to liberate his homeland and support the Warsaw Uprising, but Allied pressure redirected them to the Western Front.

Operation Market Garden was a bold attempt to push into the Netherlands and bypass the Siegfried Line from the north. Its overly optimistic planning was openly criticised by the blunt Sosabowski, but British officers, already reluctant toward him, ignored his warnings.

The Polish brigade was dropped south of the Rhine near Driel on 21 September 1944, under heavy German fire and poor weather that scattered many units. Despite this, Sosabowski’s men fought fiercely, securing the area and attempting multiple crossings of the Rhine in small boats to reinforce the surrounded British 1st Airborne Division at Arnhem. Lacking adequate equipment and suffering heavy losses, the Poles held their ground, allowing hundreds of British troops to escape, though the Arnhem bridge remained uncaptured.

Despite his leadership and the bravery of his troops, the operation’s failure was blamed on him in General Montgomery’s infamous telegram. Under pressure from General Browning, Sosabowski was dismissed and spent the rest of his life in disgrace, taking various manual jobs in Britain, while his homeland remained under Soviet rule.