Among science fiction stories with space flight, the overwhelming majority are about combat, both between spacecraft and between futuristic ground troops.
This is just the natural continuation of the process of militarisation of space Which naturally leads to questions about the space branch of the military of various nations. Some may start out as a subdivision of an existing branch and eventually grow large enough to split off (such as how the US Army Air Corps spit off to become the US Air Force in 1947 ).
"And the Navy has chosen to put more size, technology, and money into a smaller number of ships as a matter of policy.
And our aggregate tonnage is still almost three times China’s." But Naval War College professor James Holmes said there are valid concerns about the Navy’s assets. It’s more complex than just looking at the sheer number of ships, he said.
Some may grow large enough to absorb other branches of the military, others may be reabsorbed into other branches.
In William Keith's Galactic Marines series one of the themes of the early novels is how the US Marines fight being absorbed or eliminated.