The Army and the Navy cannot do what they once could and might soon be required to do again. They don’t have enough soldiers and enough ships. Even reduced to the lowest force levels in years, the Army, as USA Today reports:

... is nearly 14% short of the recruits it will need to fill its ranks, marking the first time in six years — and only the third in the last 20 — that it may fall short of its recruiting goal for the year.

And the Navy does not have enough aircraft carriers to keep one on station in the Persian Gulf.  As Navy Times reports:

When the carrier Theodore Roosevelt leaves the Persian Gulf this fall, U.S. Central Command will be without a flattop for as long as two months even as airstrikes continue against the so-called Islamic State militants. 

And those militants of the “so-called Islamic State” are not the only threat out there.  Russia and China must be accounted for.  Plus the unanticipated humanitarian mission.  The U.S. military is famous for its “can do” culture.  But there is a limit to what you can do if you don’t have the weapons and the personnel.