The reason why the congress had to pass the resolution was due to the fact that the army can only be sent on a fight abroad based on a declaration of war by the congress of the United States.
The congress has the power to declare war in the United States. It is only after they have done this that the president of the country can send the forces into war.
Statutory authorization takes place after the territory or the possessions of the country has been attacked in a foreign land.
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