How to make a fictional military force internally consistent

What makes the military forces in a fictional setting feel consistent?

Its capabilities and organization should remain intelligible, its tactics should adapt to the situation, and its ships and weapons should fit the setting's established rules.

A fictional military loses credibility when capabilities appear or disappear whenever the plot needs them. Ship design, procurement, and uniforms should show enough consistency to belong to the same force, although ordinary variation and changes over time are reasonable. Consistency also requires adaptation: a division's composition can change with the situation and the war it faces. A Byzantine example illustrates the mistake of giving an army the same organization for very different terrain. Fleet structure should also follow the setting's own science: integrated defenses suggest a role for small escorts, while defense based on distance suggests standoff weapons.