There were too MANY warning signs... half the world was already at war, and it's very unlikely that the signal of any legitimate intel that Pearl Harbor was about to happen could have risen above the noise of everything else the intel community had to keep up with at that time. How many incorrect tips and hints do you think came through, and to whom, and from whom? It's easy to find that needle in a haystack with 70 years of hindsight, but at the time, with everyone gathering intel but without the technology we have today for ferreting out useful information, I doubt it was possible to give any real warning. Besides, any attack on our land would have brought us into the war... if a conspiracy truly wanted that, then they could have given just enough warning to still witness the attack, but be alert enough to not cripple our navy and waste a huge percent of our might going into the war.
Getting back on topic, though, this IS why modern spy agencies exist. Heck, I wouldn't be surprised if everyone we spy on is fairly cognizant of it without Snowden's revelations (which, I still point out, are going far beyond the "exposing domestic spying" that is the core of his defense). When Obama was quoted in his "All nations collect intelligence" comment about people knowing what he has for breakfast, I think he was conceding that other countries, even allies, keep abreast of our leadership's activities through all means necessary. Is that right? I don't know... some of it depends on the methods and some on how deeply we go even when that leadership is trying to be private, and how much we truly share about these activities with our allies. That is, while it may be easy to know what the president had for breakfast, and while we may not even mind that that information gets out, it should be difficult to know what he discusses in the situation room or the oval office with doors closed. Snowden provides no context, no background, and no well-balanced research that traditional journalism should provide, and he and his information are losing a lot of credibility, at least with me, in so doing. Of COURSE we collect information on foreign leadership. So does the entire media population. If a crisis breaks out you can bet that some reporters will know exactly how to get hold of any leader they want to almost immediately.
From the article, it's clear that whatever documentation Snowden provided is not sufficient to prove anything, and even if it were a red-handed case (which it's not), there's still the possibility that it was done with some level of consent or coordination with Germany. The US, Germany, and Britain do actually have pretty strong diplomatic ties... something also confirmed by Snowden (http://www.theguardian.com/uk-news/2013/nov/01/gchq-europe-spy-agencies-mass-surveillance-snowden). A quid-pro-quo trading of this sort of information isn't out of the question, and could certainly make the legal aspect more difficult to unravel. This stuff is all far from black and white.