Yes there is a fundamental difference between police and military forces. In Vietnam there was an explosion of subversion within the armed forces, but hasn't the change to the "all-volunteer" professional army changed this?
Evangelical and other far-right forces have (counter hegemonic?) influence in large swathes of the armed forces.
I'm not as pessimistic as you. The unions are deeply embedded with American imperial power, but domestically at least they do advocate for working people with virtual social democrat advocacy.
Given the state of struggle in the United States I'm not sure if much more can be expected from mainstream unions.
The struggles at Republic, Stella and elsewhere has shown us that we still have some radical grassroots unions though.