To declare War: 

This is the most important power of government, and the Constitutional Convention saw fit to place it with the legislature.  For virtually all of history the powers of war and peace were assumed to lie with the executive (e.g. a king).  The Constitution splits the powers of war and peace.  Diplomacy is largely the work of the executive (see Art II, Sec 2).  But the ultimate power, to declare war on another nation, was given to the legislature to make the decision more democratic.  It would be the representatives close to the people that make the decision, not a single person elected by the entire nation. Congress was to be responsible for deciding when military force was to be used, while the executive, as commander-in-chief (Art II, Sec 2) would be responsible for day to day direction of the war effort.  In practice the president has been much more active in war making than most of the framers ever intended.  While the nation has used military force countless times, Congress has only declared war five times (War of 1812, Mexican-American War, Spanish American War, World War I, and World War II).  Since at least the presidency of Thomas Jefferson, presidents have asserted an independent war power.  Teddy Roosevelt declared that the president could use troops without congressional approval if American citizens were in immediate danger.  Harry Truman (Korean War) and George H. W. Bush (Gulf War) claimed United Nations sanction for their actions.  Far from asking for Congress to declare war, Truman and Bush asserted that an international body could grant the president power to make war.  After the Vietnam War, Congress tried to reign in presidential war making through the War Powers Act (1973).  This act essentially grants the president 90 days to make war without congressional approval (he gets 60, but can add a 30 day extension), but he must have congressional approval by the end of 90 days.  Also, the act calls for greater presidential consultation with Congress regarding military action.  This act has largely been a failure.  By giving the president carte blanche for 90 days, the act sets up a situation where the in order to counter presidential war power the Congress will have to act while troops are in the field, not before.  It is politically difficult to act against a president who has put troops into action.  It appears to be an act against the fighting men and women of the military and thus unpatriotic.  While there are periodic calls from Congress to address this situation, the president has great institutional advantages on this matter and likely his war making powers will not be decreased in the near future.