Reviewing the history of the gradual decline of the British Empire, I've noticed certain parallels between that process and the U.S.'s more recent trials in some of its foreign involvements.
For the Brits, for example, there were several occasions when the government felt it necessary to declare a deadline for its withdrawal from its colonies, or from other trouble spots that developed from its colonial experience -- India, for example, and in Palestine.
As Dr. Patrick Allitt observes, the problem with this approach is that announcing a deadline may simply delay or completely thwart the outcome the withdrawing party wants, because the other party recognizes that it will have a freer hand after the other party is no longer present. By withdrawing, one loses control, or any opportunity further to influence the coutcome.
Indeed! We should let that be a lesson to us in announcing withdrawals from Vietnam, Iraq, Afghanistan, or anywhere else. The lesson seems so clear ...
Why, then, have we so often in recent times nevertheless announced "dates certains" (as the French call them) for our pulling out?
It's the pressure of a number of modern-world factors. The main one is that as a nation, we simply have no staying power. War not over in six months? Fuh-gedda-bout-it! And then people are fed up. The electorate has little stomach for a long war, particularly in the age of instant gratification.
The others are corollary: A constant drumbeat from the media, and the concern of politicians that they'll lose favor (and elections) if they don't throw in the towel. Plus, in recent times, the cynical willingness of either major party to capitalize on such situations by joining the baying in favor of pullout - on a timetable!
Thus, a paradox: Wise diplomacy requires us to avoid specific deadlines; domestic politics demands it. This was less of a problem in Truman's or Nixon's time, but things have changed. Firm leadership in international affairs has become more and more difficult in recent decades.