Thanks for the discussion, everyone, which is getting very interesting.
there's certainly a lot of merit to quitting while you're ahead, but, it's not always quite so simple.
Not always, but in Napoleon's case I think it was. Perhaps things would have gone badly for him if he'd been more cautious, but things definitely went badly for him as a direct result of his action, and it's the consistency of his responses—attack X, attack Y, attack Z, all the while trying to increase his power—that lead me to believe that his personality was such that he didn't really understand the idea of reacting to problems in any other way, in particular the idea of withdrawing. Whether or not withdrawing or simply aiming for more limited gains would at some points have been sensible (and it's a fair point that sometimes it wouldn't have been), my point is that he wasn't, as a person, capable of doing it; he always aimed for bigger and bigger gains.
Both Portugal and Russia were thumbing their noses at Nap's complete hegemony. Portugal was small potatoes, but they refused to join the continental system. What kind of boss who needs to be feared let's a little nobody get away with that kind of insubordination?
Disobedience was a direct result of Napoleon essentially trying to centralise power over the hegemony that he had created in Europe (since the Continental System applied Napoleon's policy in the states he had subjugated, in spite of their own interests that were strongly to the contrary) for his own ends (defeating the British). He wasn't content to rule that hegemony in the looser manner of before, or to be master of most of mainland Europe while the United Kingdom remained in charge of the far smaller British Isles, even though there's no way the UK could have defeated him without him antagonising the rest of Europe too. It wasn't the British threat that caused him to want to defeat the UK; it can't have been, unless he ludicrously overestimated the threat posed by the UK (and overestimating his enemies was not exactly a trait of Napoleon's); it was his ambition.
The intervention in P led directly to trouble in Spain. Spain was getting restless with the French traipsing across their country and were going to explode sooner or later. Isn't it better to take charge of the explosion rather than react to it?
I don't think that's so. The invasion of Spain turned the Spanish against him. He ought to have predicted that by what happened when foreign armies invaded newly-Revolutionary France, but, as
Tocomocho so aptly pointed out, he didn't apply what he knew of French nationalism to the fact that other nations had nationalism too.
Russia was technically an ally, but was even worse of an ally than Spain. They did as little as possible holding up their end militarily, and even worse, dropped out of the continental system while obviously gearing up to actively oppose Nap. Is it a wiser course to take a wait and see approach to a very large country drifting back into the enemy category while it gets stronger?
Wiser than his OTL course of action? Yes. If Russia attacks him, he fights on his own ground, with short and defensible supply lines, and he hasn't lost all his army on a futile quest into Russia. We see his love for the strategic offensive and his inability to comprehend the huge advantage of the strategic defensive.
Also in regard to a later point of yours: I'm not saying he should necessarily have been able to foresee what would happen on the Russian campaign. But once he was marching into Russia and he saw what was happening (i.e. his army was losing more and more men for very little gain, and he was evidently not going to succeed in forcing the Imperial Russian Army to fight a battle) he had no practical excuse for not stopping. That, in my opinion, was a refusal to recognise what was happening in front of him and to consequently give up on an unattainable objective (the defeat of the Russian army and subjugation of Russia) because of his pride, and it led directly to his downfall.
So, an argument can be made that while France was indeed at the zenith, there were signs that things could start to swing the other way.
A fair point, but it was Napoleon who guaranteed his own defeat. In reacting to perceived possible threats (e.g. a war against Russia, a war against Spain) he turned those threats from possible to certain, he convinced the rest of Europe that he was an unstable warlord who couldn't be trusted to live in peace with them under any circumstances, and he also provided his enemies with the ammunition (surely useful for nationalism) that it was he who had invaded them. Maybe he would have fallen anyway if he hadn't done all this, but by doing all this he guaranteed his fall.
Nap was obviously an ego driven, aggressive personality, and he obviously made some big miscalculations, but it doesn't automatically follow that simply quitting while ahead leads to staying ahead.
You're right, that doesn't follow. But I'm not contending that it did follow. What I am contending is "he was never content with what he had, but always pushed for more. He didn't stop while he was ahead; it simply wasn't in his nature."
So I'm saying that this trait of his necessarily made him fall, not that he would necessarily have not fallen if he hadn't had this trait.