You ask the question as though he had a choice - he didn't.
Contrary to the mainstream belief that Hitler started the war and thus could have ended it anytime he chose, the fact is, he tried to *avoid* the war as much as he could. No one would talk with him - they were obsessed with destroying Germany and "peace" was never an option on the table.
The conflict between Germany and Russia involved Russia wanting to take over Germany to expand Communism - first to Germany then to the rest of Europe. For Hitler, fighting against Russia was a matter of survival - he had to prevent Communism from gaining a foothold in Germany and the only way to do this was by destroying it at the source - Moscow.
Even the Poland invasion, which was the historical catalyst for WW2, was over and done with for 8 months before Britain made good on their deal to (haha) "protect" Poland in the event of an attack from "some European country" (It's interesting how Russia attacked Poland a few weeks later with Britain never so much as breaking a sweat over it. But then, Russia must have already been discussing plans to support the Allied side this early in the game).
Britain had no intention of keeping their word with Poland - they only wanted Poland to stop negotiations with Hitler over the Polish corridor issue to get Hitler irate enough to start something. Once Germany attacked, Britain left them on their own. It was a matter of Britain declaring war on Germany, but postponing it for a more convenient time - such as when Britain's peace-maker Prime Minister could be exchanged for a warmonger Prime Minister.