What nation was the first to stop the German blitzkrieg and force the Germans to retreat?

Answer 1

Australia.

The Australian 9th division ("the Rats of Tobruk") pulled back to Tobruk to avoid encirclement after actions at Er Regima and Mechili and reached Tobruk on 9 April 1941 where prolonged fighting against Rommel's German forces followed, the Australians held the garrison and repealed the Nazi war machine.

This is the first documented account where the German blitzkrieg had been stopped and had to retreat.

The defenders of the fortress consisted of the Australian 9th Division, the Australian 18th Brigade and some British tanks and artillery. They were later reinforced and replaced by the British 70th Infantry Division, Polish Independent Carpathian Rifle Brigade, a Czechoslovak battalion and a British tank brigade. The siege lasted until December, when Operation Crusader pushed the DAK(Nazi's) and Italians back out of Cyrenaica.

Answer 2

The first German defeat of WWII was suffered at the hands of the British when the Luftwaffe's attempt to destroy British air power as a prelude to invasion failed. Germany had to cease its large scale attacks on British air bases and abandon its plans to invade, but continued terror bombings on London while redeploying its ground forces for Operation Barbarossa - the invasion of the USSR.