The power to wage war within the constitutional system of the United States is only one aspect of the tension between the President and Congress. Both fight to expand their influence on the political conduct of the country. In this matter, the Constitution is vague and its interpretation has caused some conflicts; however, the political regime that she consecrates gives the President more expeditious mechanisms and flexibility to make their influence prevail in front of Congress.
The president of the United States "has the constitutional power not only to counterattack against any person, organization or State suspected of being involved in terrorist attacks against the United States, but also against foreign States suspected of hosting or supporting those organizations." It is a "constitutional power", which was "collected by Congress" as "inherent to the Executive" on September 14, 2001, just three days after the 9/11 attacks. This is possible without requesting authorization from the congress. As happened when Barak Obama, in March 2011, ordered to bomb Libya.
It is true that Section Eight of Article One of the Constitution specifies that the Congress (formed by the Senate and the House of Representatives) will have, among other powers, the "declare war (...) and adopt rules concerning to the capture of lands and waters ". But it is also true that the text does not specify what a "war" is. For this reason, some other conflicts are considered "military confrontations", but have been authorized by Congress, such as Vietnam, the Gulf War, the invasion of Iraq and the Afghanistan War.