To me, you have to ask a couple of things in order to answer this question.
First, you have to ask whether America is more likely as a country to be invaded and destroyed or weakened in a serious way. I do not think you can argue that we are. There is no serious threat of invasion. Our overseas wars have not made us more likely to be attacked by nuclear weapons.
Second, you have to ask if we are more likely to be killed here in the US as a result of these wars. There is no real evidence that we are. There have not been any successful terrorist attacks within the US that you can really say were caused by our being involved in wars overseas.
I think that an argument can be made that while national security has not been immediately been sacrificed, focus has not been as prevalent on national security as it could be. The unforeseen realities in Afghanistan and Iraq has contributed to paying more attention to these realms. This attention could have been focused on national security. By this analysis, one can argue that national security has not received as much attention as it could have been receiving. At the same time, there is a certain level of puzzlement as to how to assess whether or not national security has been weakened. I think that it is difficult to assert any strong claims in answering the question. At best, informed analysis is all that one can suggest to such a question.
In the long term, yes. As U.S.A. attempts to extend and strengthen its empire, it becomes thinner in its coverage, or it has to divert resources at home from wealth production, into its national security establishment.
I believe the question relates to wars being carried out by USA.
It is not possible to assess with certainty the effect of this kinds of war by USA or by any other country, where the the objective of the war is not to protect the country from some direct and immediate security or other threats.
This assessment is particularly difficult before the war is taken up, though these may become clearer as the war progresses. This is because the course and consequences of such actions do not always follow the predicted path, and because the objectives sought to be achieved are direct rather than indirect.
However, one thing is clear that a country initially takes up such action only to protect its interest and to strengthen its position. In the end whether its objectives are achieved or not is quite another matter.
Considering all these factors into consideration, It appears to me that overseas actions of USA in countries like Korea, Vietnam, Kuwait, Afghanistan and Iraq may not have been fully realized, but most certainly these have not weakened the country. It appears to me that these wars have actually helped to established the position of USA as the undisputed top military power in the world. And most certainly these wars have not affected adversely the national security.