The single greatest evil of the Bush presidency...
...was his popularization of the notion that preemptive war was somehow an acceptable thing for Americans to engage in. It is a tragedy to see so many Democratic leaders, and so much of the Democratic base, still drinking that particular Kool-Aid.
Preemptive war has a dark and bloody history, and has traditionally been the tool of the "bad-guys". Who embraced preemptive war prior to Bush? Well, the Japanese used the argument to justify their invasion of China, which Japan claimed was positioning itself to invade Manchuria. Hitler used it to justify the invasion of Norway, claiming (correctly, fwiw) that the Allies were going to use it to invade Germany. In 1837, in an event largely forgotten by Americans, Britain crossed into the United States to capture and kill some Canadian and American rebels who it believed were planning to attack Britain. And that's largely it in recent history. Even Reagan wasn't willing to go THAT far. Until the advent of the Bush Doctrine, the concept of "preemptive" war was abhorrent to most of the world. Luckily for us, in the rest of the civilized world, it largely still is. America seems to be alone in its ethical march back into the dark ages.
Following the Caroline Affair (the British cross border attack of 1837), Secretary of State Daniel Webster established the Caroline Test, a standard for judging the acceptability of preemptive strikes against another nation. Under that standard, a call for preemptive war had to meet four criteria. The need had to be "instant, overwhelming, leaving no choice of means, and no moment for deliberation". The Caroline Test was enshrined into International Law, and was reaffirmed at the Nuremberg trials after WWII. It stood as the baseline requirement for military action in America from 1842 right up until the Bush Doctrine replaced it, and was one of the founding principles of both the League of Nations and the United Nations.
The Caroline Test:
Instant - The need to attack must be imminent. You cannot attack an enemy simply because they may be a threat at some point in the future.
Overwhelming - There must be no question about the danger that is posed. A nation must be completely sure of an impending attack before it launches a first preemptive strike.
Leave No Choice of Means - War must be the only option. If isolation or diplomacy is an option, war is never justified.
No Moment For Deliberation - Related to the first, but also meaning that the use of alternatives to war will simply lead to the other nation striking first.
The Bush Doctrine, taken directly from George Bush's own mouth:
"If we wait for threats to fully materialize, we will have waited too long... a military [that] must be ready to strike at a moment's notice in any dark corner of the world. And our security will require all Americans to be forward-looking and resolute, to be ready for preemptive action when necessary to defend our liberty and to defend our lives." "The liberty we prize is not America's gift to the world, it is God's gift to humanity." The defense of freedom requires the advance [military promotion] of freedom."
By embracing a preemptive war simply because a group MIGHT be a threat to the United States in the future, Obama has aligned himself with Bush, and rejected the values of nearly every American President before him. The Bush Doctrine, which may also be rightly be called the Obama Doctrine now, is a rejection of the values that have defined America's role in the world for much of our history. It's a rejection of the international system, and the expectation that war will always remain an act of final resort. It's a rejection of human dignity, of the notion that all human lives have the same value, and ultimately of peace itself.
I may not be the most liberal person on the planet on every single topic, but when it comes to the pursuit of peace, to the furtherance of the notion that war is an intrinsic wrong that turns our children into the agents of death and destruction upon the Earth, I have been unwavering. If that makes me part of the "isolationist left", then so be it. If that makes me sheltered, or pampered, then so be it. Peace cannot be achieved through war, no matter what Bush would have liked us to believe. War only achieves more war. Death only begets more death.
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He tried to make it sound that because he had a neat description for it (preemptive war) that it was a legitimate reason for it. Thank God the press called him on it! 
