Maybe-relevant current events thing: Obama said himself that he doesn't have the constitutional power to do this military-involvement-in-Syria sort of thing (in the context of a hypothetical pre-emptive attack on Iran's nuclear capabilities, a scenario wherein it was explicitly stated there was NOT an imminent threat). Not without authorization by Congress.
That gets my 
But now he's acting like he may go and do it anyway 
President can move troops and conduct military operations anywhere in the world so long as he informs congress within 48 hours?. Also the War Powers Act is still on the books allowing for the President to conduct military operations for 60 days without the consent of congress. Followed by a 30 day period for troops to withdraw. Presidents are usually given very very broad ....leniency in the use of this power. Congress has not issued a formal declaration of war since WWII, but they have approved every conflict via a resolution (correct me if wrong). Also the Pre-emptive strike notion seems...unlikely in the manner that most people thing of as a sledgehammer destroying everything. So long as casualties among the US military remain low (like Libya) chances are both the public and congress will go along with limited intervention.
I do believe the current situation in the US regarding the use of armed forces reflects a crisis of national identity in regards to how the US should conduct itself in foreign diplomacy post Cold War. For ~half a century foreign policy was defeat the soviets (This is somewhat simplified). I think that the US is currently struggling to define itself on the world stage in what it wants to do accomplish without an adversary. It has the most powerful military in the world and nothing to do with it. There is a void and its purpose must be filled as "defense" is no longer a legitimate reason for the size and power of our current military. Some people/views would like the US to act as the world's police force/peacekeepers (moral obligation), another idea is to extoll/export american virtues (Freedom, Democracy etc), another is to see the expansion of US interests abroad (economic, military), another view is that the our military is unnecessary in its current state and should be downsized. Lastly there is a an isolationist trend that wants to let the world do its own thing and focus on issues at home. It should be noted that none of the these forces are mutually exclusive and the arm of American influence is motivated by all of these factors at time.
Additionally I believe that America since the Monroe doctrine has been an imperial power, and since the turn of the 20th century has been a non-traditional empire in that it does not seek to acquire territory, but instead to acquire influence. I also think America has undergone a slow decline of it's world influence since the end of the cold war, but policy makers in many ways still act as if the US has more world influence than it currently does.
