Answer:
Congress serves as an important check.
Step-by-step explanation:
If the president is allowed to send troops wherever without consent from congress, things could get dreary all the time. Not only would the soldiers be suffering from constant deployment, but the country's economy would shatter.
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If this still doesn't make sense here's a better example:
Let's say a bloodthirsty president comes into power. All they would want is power, and if they had the authority to send troops whenever they felt like it against countries, so many people would die and their country would keep getting bombed by forgein bombs. It would be absolute chaos.
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I hope I helped :)