The last few wars have been declared by Presidents without an act of congress. Yet the Constitution says that only congress can declare war.
What is the specific legal justification that Presidents now use to declare war?
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First of all, it should be clarified that no President has declared war at any stage. This, as you mentioned, is within Congress's power.
The War Powers Resolution of 1973 requires the President to notify Congress within 48 hours of military action being taken, and prohibits this action for continuing beyond 60 days without the authorisation of Congress, by means of either an Authorisation for Use of Militar Force (AUMF) or a declaration of war.
The power to declare war has never been transferred, it is just that the world order has changed, and this has left some aspects of the US constitution obsolete.
So, just as the constitution demands "The number of Representatives shall not exceed one for every thirty Thousand", no modern congress is ever going to get near to the 9000 limit implied by this restriction. Nor will there ever be a class of Americans that don't pay taxes or have representation in Congress (the so called "Indians not taxed")
Likewise, the world order, post WWII, with a United Nations, and nuclear armed superpowers means that the system of war that existed in the 18th century doesn't exist any more. In the 18th century, and until 1945, a country had to either issue an ultimatum, or a unilateral declaration of war before hostilities commenced. Failing to do so would harm your chances of forming alliances. But if you did declare war, then the war would be lawful according to the traditions of international affairs. The constitution expects this system to exist and demands the Congress has the right to make such declarations.
The establishment of the UN changes this, as under the rules of the Security Council, offensive war is not allowed. A country cannot lawfully declare war on another country. Moreover it was previously understood that a country would use all the weapons at its disposal in procecuting a war. Nuclear firepower has changed that. All wars since 1945 have been limited conflicts with the nuclear nations choosing not to use a whole class of weapons. So now there are no declarations of war. The words continue to exist in the Constitution, but they have no more consequence than the notion of "Indians not taxed" or the limit on numbers of Representatives.