Sure, its easy to rest on your laurels, but a lot harder and more expensive to catch up once you figure out the competition has not been. The idea is to be superior to your foe, not equal. China under reports their spending and does not have to be concerned with power projection in two oceans.
That depends on your definition of a conventional war. If you are talking state actor on state actor action, then maybe 1973 in Vietnam, but that was really a conventional war victory and a insurgency defeat. So perhaps 1920 backing the White Russians? Really, in state actor on state actor wars, the US has done very well.
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The F-35 is intended for near-peer conflict, not for performing land occupation or COIN. The US military is not designed to be an occupational force, nor is the US generally willing to expend the money needed to win a insurgency.