Benjamin Friedman notes how quickly the new intervention in Iraq has expanded:

Monday, the President again broadened the bombing’s objectives. The airstrikes against ISIS still protect U.S. personnel and serve humanitarian purposes, he said, but now, it seems, those are general goals that ongoing bombing serves. The President also suggested that ISIS is a security threat to the United States. Not for the first time, he said that once the new Iraqi government forms, we will “build up” Iraqi military power against ISIS.

Only the speed of this slide down a slippery slope is surprising.

This is why I said two weeks ago that the intervention would last much longer than originally advertised, and it’s why I said last week that I had no confidence that the administration would avoid expanding the mission in Iraq to include additional goals. Not only has this administration proven in Libya that it will expand the goals of an intervention once it has started, but it is almost impossible in practice to adhere to the original restrictions that are supposed to keep the mission limited in the first place. These are restrictions that the executive pretends to impose on its own behavior, and there is almost no one interested in holding a president accountable for ignoring them.

If anything, the conventional wisdom in Washington is that it is wrong for a president to rule out any option, including the use of ground forces, which means that a president is more likely to come under attack for keeping an intervention “limited” than he is for escalating U.S. involvement. “Limited” intervention isn’t possible for the same reasons that the U.S. so often opts for “doing something” instead of staying out: there is a bias in favor of action in our foreign policy debates, there is excessive confidence in the efficacy of hard power to solve problems, there is no meaningful institutional obstacle in practice to presidential war-making, and the people with the greatest interest in the issue are always overwhelmingly in favor of doing more rather than less.

I have seen arguments that say that Obama is the least likely recent president to “allow” mission creep, but that misses the point entirely. Mission creep doesn’t have to be something that a president wants from the outset, but comes about because of what happens in the conflict after the U.S. joins it. These things tend to take on lives on their own, and once a president starts down the path he is pulled along by both success and failure. All the while, he is urged to “finish the job,” which usually guarantees that the “job” will never be finished because it keeps growing in size. In that way, what starts off as a “limited” intervention keeps growing in ambition until the goals become unrealistic and the U.S. commitment becomes open-ended.

Why does this keep happening? Once a president has committed to using force in a foreign conflict, all of the effective political pressure is on the side of escalation. Having conceded that the U.S. should be involved militarily in a conflict, the president is bombarded with demands for deeper involvement in order to pursue the illusion of victory. If he doesn’t agree to these demands, he will be steadily pilloried in the media until he does, and any adverse development in the affected country will usually be attributed to insufficient American involvement. Since the initial decision to intervene was driven in part by the same sort of pressure, it is more than likely that the president will keep yielding to calls to “do more.”

Once an intervention begins, the politically easier route is to continue it whether it is perceived to be “working” or not, and even if an intervention is perceived as failing there is a perverse incentive in our political culture to throw more resources at the problem and persist in the policy. Politicians from both parties are firmly opposed to admitting that their preferred policy has failed and that the time has come to cut our losses. If the intervention enjoys some initial success, that can be even worse for leading to an expanded mission, since it encourages a president and his allies to become more ambitious in what they hope to accomplish. Eventually, the mission lasts long enough that it is added to the already extraordinarily long list of foreign commitments that the U.S. cannot “walk away” from for fear of lost “credibility,” and the longer the mission lasts and the more that it costs the larger its goals have to be in order to justify the effort that has been made.