The United States is the safest country in the world. It has vast ocean moats to the east and west and weak friendly neighbors to the north and south. It has sufficient resources to shun global trade should it so choose. It is also the wealthiest and strongest country the world has ever seen.

The heaviest foreign policy costs it has paid in the 20th and 21st centuries have come from self-inflicted wounds. The war in Vietnam. The war on Afghanistan. The war on Iraq. Those have had terribly high human, economic, resources, moral, and opportunity costs. Each of those wars was a choice made by U.S. presidents.

With the resignation of Secretary of Defense Mattis some have become more concerned about competitor states taking advantage of the chaos within the U.S. administration. These concerns are real, but unfunded. For example, if Russia invades Ukraine, which is highly unlikely because Russia has no need to do so and knows that holding Ukraine would be costly and probably beyond its capabilities, it is no skin off the U.S. nose. The United States could choose to help its European partners or not and remain safe either way. China likewise has no need to invade Taiwan. Ties have been growing ever close for some 20 years. China knows it has no need to use force to attain what it will gain peacefully with time. Further, North Korea has no need to start a war against the South. North and South Korea have been getting along quite well recently, in fact. It is the United States that scares North Korea, and rightly so, given U.S. bluster. The North Koreans have good reason to keep their nuclear weapons to defend themselves against any possible U.S. effort at regime change.

Sending in U.S. ground forces in any of these scenarios, to Ukraine, Taiwan, or Korea, would be a costly mistake. First, because the United States lacks the will and capability to end any fighting or permanently force back the aggressor. Second, because U.S. forces would probably face a nasty nationalist backlash. Third, because the use of U.S. force in these three cases would not help achieve the political objective the United States would presumably prefer, namely a return to the status quo ante bellum.

There are many things that U.S. forces can achieve. There are many more things that they cannot achieve. The key to success via the military tool is to distinguish between the two. The United States could lead or contribute to negotiations to end the fighting in any of these scenarios, if the president could find appropriate envoys. There are surely more diplomats, military officers, and businesspeople willing to fall on their swords to serve U.S. interests by serving in this administration. But resolving any of these potential problems is not up to the United States and there may be other actors better positioned and with stronger leverage.

Make no mistake, this administration can cause significant domestic and international disruption. But Mattis has done all he can. The perfect replacement would face exactly the same constraints that Mattis did. The president’s views are likely to prevail, to a greater or lesser degree and for greater or lesser harm to U.S. interests. This is how civilian-military relations are supposed to work in the United States, no matter how distasteful some may find the executive’s views.

The chief impediment the president will face in replacing him is, first, finding someone to agree to serve in this administration, and second, finding someone whom the Senate will confirm. There is not a large group of people at the intersection of that Venn diagram. If the administration ends up with something like an acting SecDef for the next two years, well, the United States is strong enough, rich enough, and safe enough to muddle through – under most conditions. My major concern is the possibility of unnecessary escalation caused by U.S. choices in an international crisis. Whoever fills the role of SecDef at that time, along with his/her colleagues, their options will be limited by the president’s gut.