Why does only the Congress have the right to declare war?

As big as the office of the president is, the president doesn't have full right to do some things. Many people still think the president is mightily empowered by the Constitution to do anything. A good example of some of the things the president cannot single-handedly do is declaration of war, though the president is still the commander in chief of the Armed Forces.

So when the state is being threatened by external forces, the president can only declare a war only when there's an approval or a Constitutional backup from the congress. The power to declare war is vested in the hands of the Congress. The fact is the power to declare war is only in the hand of the Congress but that does not mean the president will always have to wait for an approval from the congress before he can carry out his constitutional responsibility to protect every citizen of the country.

There is some special case to this power of the Congress. For example, when there is an invasion by any enemy group on the US soil, the president is duty-bound to declare a war on them without an approval from the Congress.