Hitler hoped to secure the southern flank of Eastern Europe in preparation for his campaign in Russia by invading the island of Crete. On May 20, 1941, “Operation Mercury” commenced when German paratroopers were dropped onto the island. They met fierce opposition by Greek, British, and New Zealand troops who knew the attack was coming, thanks to the deciphering of German messages.
Though the paratroopers initially suffered heavy casualties, the tide of battle turned when German forces secured an airfield to fly in reinforcements and offer a base to the Luftwaffe. Afterward, the resistance crumbled. By the end of the month, the Allies were forced to evacuate, suffering roughly 20,000 casualties compared to 4,000 dead for the Germans. In contrast to the gentlemanly treatment of the Greeks during the German occupation of Greece, the Germans adopted a scorched earth policy in Crete, destroying almost everything in their path and murdering Cretan civilians.
Despite taking the island, Hitler saw the battle as a fiasco and lost faith in the paratroopers under General Kurt Student. He never ordered another major airborne attack for the remainder of the war. Of course, Hitler didn’t know that the paratroopers had failed in part because the Germans lost the element of surprise after the British intercepted their plans.
Sources: Bard, Mitchell G. The Complete Idiot's Guide to World War II. 2nd Edition. NY: Alpha Books, 2004.