Public opinion has turned against every American military engagement that has lasted more than a year with the exception of World War II. The reason for this is fairly straightforward: a good number of the majority who supports intervention at the outset has not factored into their thinking all the eventual costs and consequences of the campaign [bold mine-DL]. Eventually, the accumulation of costs—be they casualties, increased terrorism or the economic toll of war—will start to overwhelm the initial support, especially for those without particularly strong reasons to support the war in the first place.
It is impossible to imagine this campaign avoiding a similar fate if it indeed stretches out to three years or beyond.
One of the reasons that many early supporters of military interventions don’t factor costs and consequences into their thinking is that the proponents of the intervention make a point of minimizing and obscuring these from view. Like all advocates pushing a particular policy, interventionists emphasize and exaggerate the dangers of not adopting their recommendations and oversell the benefits of “action.” They typically have a dismissive, cavalier attitude towards unforeseen and adverse consequences of military action and they assume  that “there is no real harm in trying.” That arrogance and overconfidence make “action” seem appealing early on, but set the U.S. up for disappointment, frustration, and bitter recriminations later.
In most cases, the near-instant bipartisan consensus that congeals around an interventionist policy and the attendant media demands to “do something” tend to drown out countervailing arguments during the first few months of the campaign. This boosts public support for military action in the short term, but like any bait-and-switch trick it also causes people to sour on the intervention more quickly than they might have done otherwise. More Americans gradually become aware that the threat to the U.S. was overstated (or simply made up) all along, and they start to realize that the war they were originally told about at the beginning is not the one that the U.S. is actually fighting. Because presidents often set unrealistic goals for these interventions, there is usually even greater disillusionment because the war comes to be seen as “not working.” That is a trap that presidents set for themselves. They are the ones promising results that aren’t possible, and those results certainly aren’t possible at the very low cost that the public is willing to accept.
In addition to length of time, the costs of a prolonged intervention naturally drive down support as they increase. Support for military action often starts vanishing as soon as the war involves the loss of American lives or the extended commitment of U.S. resources. Another factor that makes public support for military intervention relatively fleeting is that almost all of the wars that the U.S. has fought in the last fifty years have been unnecessary ones. If a war were genuinely necessary to keep the U.S. secure from a foreign threat, a majority would likely keep backing it for a very long time, but since almost none of our modern wars falls into this category it is unreasonable to expect that there would be sustained public support for a war that didn’t have to be fought. That is especially true for illegal wars waged without Congressional authorization. Whatever the polls may say at the start of a war, the president can’t claim to have obtained the consent of the public unless their representatives have voted specifically to authorize it. The longer that a president waits to seek that authorization, the more that he and his party will come to “own” the war. As a result, it will be easier for the rest of the country to turn against it and make it very unpopular. Clinton and Obama were able to get away with their illegal wars in Kosovo and Libya despite limited public support, but those wars were over in a matter of months. Waging a multi-year war without explicit Congressional authorization and relying on obviously bogus legal arguments to justify doing so will likely make this war unpopular much sooner rather than later.