There is no necessary conflict between ethics and leadership. Any conflict that does exist will also be a conflict between ethics and other ethics, which will involve leadership on one side.
A government has an ethical commitment to protect its national interests and the safety of its citizens. The value of leadership will be on the side of this ethical commitment. Any conflicting ethical commitment will have to be of paramount importance to overrule the presumption that the government should act on the basis of national interest and safety. Such conflicts do not arise often in questions of national security.
For instance, it is quite clear that the US should do everything it can to avoid launching a nuclear strike against China or Russia. In the current situation, all the US's ethical commitments point to this conclusion. The important question is, how much would things have to change for there to be some ethical conflict? What degree of aggression on the part of China or Russia would make such an act justifiable? The invasion of an ally? The threat of a direct attack? In these cases, global security, and the lives of many Chinese or Russian citizens would have to be brought into the equation.
However, it is unlikely that there would ever be a straightforward trade-off between American and Chinese or Russian lives, and the necessity of taking one action or the other ensures that it will never be clear whether the right balance was achieved. Historians, philosophers, and strategists still debate whether it was ethically correct to bomb Hiroshima and Nagasaki. Did the bombing bring to an end a war that could not have been ended in any other way, thus saving more lives than it claimed? This question is impossible to answer, and any future question about the ethics of a reaction to a security threat from China or Russia will prove similarly unanswerable.