AN ACT to authorize the President to increase temporarily the Military Establishment of the United States. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That in view of the existing emergency, which demands the raising of troops in addition to those now available, the President be, and he is hereby, authorized—

–Selective Service Act of 1917

Why is the president being given this power?

to give the troops additional money
to decrease the power of the president
to establish a military in the United States
to temporarily increase the number of troops