The other allies (England, France, etc) were too war-weary and busy rebuilding their countries to really challenge them. Germany reneged on a deal regarding splitting conquered territory North and East of Germany with Russia, so Russia switched sides. Later, NATO and the UN would drive the wedge even further, Russia shut out of NATO and constantly overridden on the UN Security Council. Germany and Russia were initially allies, like-minded in their expansionist intentions. It was an alliance of convenience mostly.
