The Spanish-American War was the most contrived...but the War Between the States could have been prevented if Buchanan had had the guts to order the arrest of those hotheads in Charleston harbor from the start...
Not to give Buchanan that much of a pass, but he had no historical president to work with...no lessons from previous US history. I don't think he could have foreseen a bloody four-year civil war.
But Bush II and Obama both had the Vietnam precedent. The Afghanistan war is just Vietnam 2. When Bush II set up the original ROE with all that nation-building nonsense, he was simply repeating all the KNOWN mistakes of the Vietnam war.