Why cant the president declare war?

The President can't declare war because in the Constitution that power is given to the Congress as part of the checks and balances system.
No.
No. It is Congress who can but hasn't since World War II and meekly approve Presidential "actions" of one sort or another and it is the President as the Commander in Chief who can send armies into "action".
No it first congress has to pass the bill then it goes to the president to sigh. The President can ask congress to declare war, but he can't declare war congress only can.