From a strictly military point of view, no-one did more to ensure that Germany lost the war than Adolf Hitler.
His military decision-making was appalling throughout the war.
Some examples -
He went to war before Germany was ready.
He ordered the Panzers to halt for three days in May 1940 - which enabled the British to evacuate 300,000 soldiers from Dunkirk.
He delayed too long (indecision?) before lainching any sort of attack on Britain.
Just when the Luftwaffe was finally getting the upper hand, he insisted that they switch their attacks to London.
He invaded Yugoslavia and Greece, tying up half a million men for the res of the war.
He kept changing the objectives of Barbarossa. First Moscow didn;t matter - then it wwas priority number one.
He declared war in the US.
He diverted a mass of manpower and materiel from the Caucasus to Stalingrad.
After which, he lost touch with reality.....