sierra_nevada posted a lot of valuable information, and the validity of what was said can probably not be very well disputed (he has this annoying tendency to be right most of the time ;)). However, in response to all of the examples and history, I have this to say.
The US government operates in an intentionally tenuous balance between explicit trust and distrust. That is, Congress creates laws and trusts (to a certain degree) that the President will execute those laws. Yet, the checks-and-balances concept implies that Congress alone, acting on behalf of The People" has the primary power to call the actions of the presidency into question. From my perspective this design means that when the President presents information as cause for action, Congress decides on a superficial level, with assumption that the information being presented is true, whether that action is in it's own (aka, The People's) best interests. Later Congress has all kinds of powers to call the actions, information, or justifications into question... but those things take time and are inherently reactive.
In nearly all of the examples presented (and granted, I haven't thoroughly researched this per se), this was the process that was followed... as designed... and this is the case with the invasion of Iraq. The President said "this is the case," Congress said "Based on this, go forth and conquer," and when Congress learned it wasn't the case, Congress began calling things into question.
I'm not saying the executive branch is perfect... the design of the US Government attempts to directly recognize this... but the reality is that the time that it takes to be certain of something inhibits the ability to act quickly, and this is also exactly what the design of the government is designed to do.
...and it's to this end that I respect Hillary. At the time, with the information presented, she did what she thought was right. These are the kinds of things you HAVE to do in government and in life. There may be a lot of things to apologize for... for outcomes... for subsequent inaction... for not learning the mistake... but I'll give credit for not apologizing for making the right decision at the time.
I'm not saying that we should inherently trust the office of the president. I'm not saying that congress isn't responsible for ensuring the president is acting properly. I'm saying that in the Iraq example and in all of the other examples provided, the presidency is only called into question by congress when congress has a reason to, which implies that Congress is always going to be operating in a reactive state. To hold Congress (and Hillary) accountable for what it should have known despite what it was being told by the President is in contradiction of the design of the government.