The United States is in another war for the purpose of changing another regime. Those who deny this are kidding themselves. President Obama has repeatedly said that Qaddafi has to go. In addition, military force was used against him only when it appeared the rebellion might be defeated without it; the war is part of an international campaign to force Qaddafi out; and the success or failure of the war will be judged largely by whether Qaddafi manages to stay in power.

Of course, Washington is debating whether this new war is a mistake. That question is impossible to answer with any accuracy because no one has supplied the strategic context that makes the question meaningful.

This is a point I have made before. The last four American presidents have failed to do what only they could have done: define America’s strategic mission in the post–Cold War world by identifying the national interests the United States will seek to advance and the means by which it will advance them.

In the absence of strategic clarity, foreign policy becomes a series of impulses, expressed through often contradictory decisions, in reaction to events that no one managed until they grew into crises that were not anticipated.

The United States military is one tool of American foreign policy. When there is strategic confusion, the military fights in some places but not others, based on distinctions that are at worst irrational and at best unclear.

Some examples include America’s fighting to stop cruelty to civilians in Bosnia and Libya, but not in Sudan or the Ivory Coast. America does not engage in “nation building” — except in Iraq and Afghanistan, and before that in Bosnia, and before that in Japan, Korea, and Germany. Weapons of mass destruction, particularly in the hands of terrorists, pose the greatest danger to America. Yet America, now engaged in removing a Libyan regime that ended its WMD program seven years ago, is taking no military action against an Iranian dictatorship that is every bit as brutal as Qaddafi’s, is busy building nuclear weapons, and is the chief sponsor of international terrorism.

None of these decisions was necessarily wrong. But they all were the products of impulse rather than policy, or at best they were based on assumptions not articulated and not formed into a coherent strategy. That makes it difficult to weigh the costs against the benefits, since both can be calculated only with reference to broader objectives, and, over the last four administrations, no one has formulated such objectives.

There is another problem with strategic confusion: When the government doesn’t know which objectives are important, it tends to neglect the capabilities by which all objectives are achieved.

Since the end of the Cold War, America’s military has operated at a far higher operational tempo than during the Cold War years. But while the military has been busier than ever, its size and strength have been declining. The Air Force is smaller, and its planes are older, than at any time since the inception of the service in 1947. The Navy has fewer ships than at any time since 1916. All three of the services are 30 to 40 percent smaller than they were during Desert Storm, which is the reason why the Guard and Reserves have been constantly mobilized, and why a number of Army units are on their fifth or sixth deployment in Iraq or Afghanistan.

The situation is so bad that an independent review panel headed by former secretary of defense Bill Perry recently declared that “a train wreck is coming” for America’s military unless it is strengthened and modernized.

Yet the percentage of the federal budget being spent on sustaining the military is at an historic low. The Obama administration has been busy cutting modernization programs for the last two years. Not to be outdone, Congress is passing spending bills that cut the administration’s proposed defense budget even further.

President Obama is not exactly a hawk on military issues. One would think that Congress would be reluctant to cut his defense budget, at least while Americans are dying in Afghanistan. But considerations like that seem to have no impact in Washington these days.

One gets the sense that the leaders in both parties — with a few notable exceptions — have simply given up on foreign policy. They can’t figure out how it all fits together or which parts of it really matter. So they act as if none of it matters. Their “policy” is to wait for the biggest headline or the most pressing demand, and then to react. That reaction usually takes the form of sending American troops to do more and more with less and less, for reasons that even the leaders understand are pretexts.

Unfortunately, the United States does have vital interests abroad, and the threats to those interests are growing. The Libyan campaign, however it ends, shows again that America is sailing with uncertain masters in troubled waters, trusting to luck, and neither anticipating nor prepared for the gathering storm.

Jim Talent is a distinguished fellow specializing in defense studies at the Heritage Foundation.