To quote myself from another thread:
As far as most historians can say, Nixon didn't actually order the Watergate break in. It's easy to ascribe it to him, since, he's Nixon of course he's guilty of doing it. He must've been. But there's not a lot of actual proof to back this up. There is far more evidence to suggest Nixon found out about it after the fact, and then overreacted to it. Rather than doing the right thing, fire everyone involved, and turn them over to the FBI, he sought to protect his people (and by extension himself, or so he believed) by covering it up. From there on he continued to make bad decision after bad decision, committing more and more crimes to cover up the cover up and so on. So while it's unlikely that he was guilty of ordering the break in, though, he is certainly guilty of creating the atmosphere that allowed the people under him to think doing that to begin with was okay.
The book Being Nixon: A Man Divided by Evan Thomas has a lot of good info about it. And even it suggests it's more likely that Nixon was involved with sabotaging the Vietnam peace talks than ordering Watergate (though, the book comes down on the side that Nixon's people probably did act with his knowledge, it likely made no difference in the outcome of the war, illegal as it was). It's interesting to note, though, that Nixon himself was concerned until the day he died with the idea that he may have ordered the Watergate break in and then...simply forgot about it. Seems unlikely, but when even the man himself was unsure, it makes it all the harder to say that he was guilty...of that one thing, at least.