Actually they can raise many armies (It's plural, look it up Art 1 Sec. 8), currently they've raised two. The ground Army (aka the Army) and the Air Army (aka the Air Force). They also can provide and maintain a Navy (one only, which the Marines are part of).
the duty (it says "shall" in the Constitution) isn't merely to protect our borders but "against Invasion". (Art. IV, section 4)
Typical Feds!