The blank helped the U. S recruit 4 million soldiers in 18 months

To that end, Congress passed the Selective Service Act, which Wilson signed into law on May 18, 1917. The act required all men in the U.S. between the ages of 21 and 30 to register for military service. Within a few months, some 10 million men across the country had registered in response to the military draft.

Explanation:

the constitution of the united states gave congress the absolute authority over immigration laws

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