The reason that Germany was unable to invade Britain is because they didn't have the naval power to do so. Battleships (the German equivalent. I don't have my books handy to reference the exact details although I could if anyone is that interested) take a lot of time and materials to build. The air force and army was built up rather quickly. But the naval rearmament couldn't be rushed. The commander of the navy (again. I forget details. Doenitz?) was under a time frame in which the navy would be up to full strength in the year 1945 or 1946. It was at this time that the war was supposed to begin. But Hitler couldn't resist taking his little bites and nibbles. He kept thinking that he could get away with just one more. And he was almost right. If France hadn't stood strong on their alliance with Poland, Chamberlain would have signed Poland over to the Nazis in 1939. He was trying and this trying ended his career. Churchill was a hawk and took advantage of Chamberlain's peacenik mistake to take power.
Anyway, the only way that the nave was up to par was in submarines as they were small and relatively quick and easy to manufacture.
Hitler's greed got the better of him.
It's funny, because in the early days his generals were against him. They lit him march across the Ruhr because they were sure that he'd get his ass handed to him and they'd be able to keep him in line afterwards. Same with Austria and Czechoslavakia. Each time the generals were prepared to revolt and remove him from office once things started to go bad, but they didn't. The peacenik League of Nations played into Hitler's hands and with each successive victory his hold on his nation tightened. Here was a man doing the impossible with bluff and bluster. By the time Poland came around, the generals were mostly in the palm of his hand and the people would have slaughtered the generals had they attempted to remove him from power.
And, by the way, I don't think that Stalin did think of the non-agression pact as a temporary solution. He thought he truly had an ally in Hitler and Germany. Stupid, yes. But Stalin was not exactly the brightest bulb in the bunch, you know. In fact, when Hitler broke the pact and began his attack, Stalin refused to believe the reports, saying that they were a mistake or miscommunication of some sort. The Germans advanced for a day or two before Stalin was finally able to admit that he had been betrayed. At this time, he retreated to his home and waited for the party executioners to come and eliminate him. After all, that's what he'd do in their position. And they came. But they weren't executioners. Instead they begged him to come back to Moscow and take command. The country needed him.
Stalin was shocked by this but he quickly recovered and because of this weakness in his subordinates, all the atrocities that he visited upon his own people in later years followed.
But, his actions clearly demonstrate that he believed the Nazi's would be as good as their word in this.
What's funny is that Hitler attacked the USSR because he figured that if he did so it would work to get the West behind him. Better a German ally than a Soviet one. Again. He guessed wrong. His luck had run out.