Yes, you can use the same military power bonus over and over again.
At any time, you can count up your total military power from what's in your tableau. For example, if you have Space Marines (+2) and Drop Ships (+3), your total military is 5. That means that you can conquer any military world with defense at most 5. It doesn't matter when you played each development, or what military worlds you may have played in the past; your current military power is still 5.
So I wouldn't think of it as military worlds costing military to settle. You just compare your military power to their defense, and see if you have enough. Using your military power doesn't expend it.