President Obama is about to send up to 300 military advisers to Iraq in an attempt to stabilize a situation that is rapidly disintegrating. Obama had hoped that the end of the Iraq war would be a key accomplishment of his administration. But just as he thought it was safe to get out, the President is finding himself drawn back, as violence has been spreading throughout Iraq.

Understanding that American patience for another war is limited, President Obama promises this mission will be contained.

But mission creep is difficult to avoid. The history of military involvement shows that many operations that start small end big. While the United States initially entered Korea to try to get the North Koreans out of South Korea after an invasion, President Harry Truman found himself presiding over a full-scale military mobilization that lasted three years, cost over 30,000 lives and helped bring down his administration.

Vietnam started small, with military advisers under Presidents Dwight Eisenhower and John F. Kennedy. Even when President Lyndon Johnson requested from Congress the Gulf of Tonkin Resolution that granted him broad authority to use military force, he didn't imagine how big the conflict would become, resulting in the death of nearly 60,000 U.S. soldiers and dramatically undermining America's role in the world.

Examples of mission creep continued. George H.W. Bush had 30,000 troops enter into a peacekeeping mission in Somalia. The mission didn't go so well. As a result of an attack on U.N. forces by a warlord in the country, the operation expanded and President Clinton found himself ordering more expansive operations.

Although George H.W. Bush was determined to stick to his goal of kicking Saddam Hussein out of Kuwait in 1990 and 1991, once troops were in the region, the United States became committed to ongoing engagement with Hussein as he flouted U.N. resolutions. In Afghanistan and Iraq after 9/11, the United States vastly broadened the scale and scope of its operations as challenges of post-regime reconstruction proved immense.

Why does it prove so difficult to contain operations? Why is mission creep so common? Most importantly, war inherently involves many moving parts, most of which are not under the control of the commander in chief.

Often, as was the case with South Vietnam in the 1960s, allies prove difficult to rely on and cause problems of their own, while opponents frequently are capable of causing far more trouble than expected, even when they have fewer resources than the United States...