Of course the first would be invading Iraq. But a close second not often remembered or even realized occurred two months into his term of office. It was the humiliation inflicted on then South Korean President Kim Dae-Jung, a Nobel Peace Prize winner and former torture victim of earlier military dictatorships in that country, when he arrived to visit the US. Secretary of State, Colin Powell, had planned on a continuation of the warming policies between the two Koreas and a friendly meeting with Bush and support for Kim. As it was, the "Vulcans" led by Cheney and Rumsfeld intervened to torpedo this and leave Kim not meeting in the cold and without his long-running policy. This also ended that round of negotiations with North Korea on nuclear weapons, with the Vulcans arguing that the US should pursue "regime change" in North Korea, which, hack cough, did not happen.
Instead what happened was that the nuclear negotiations came to and end and US-South Korean relations tanked until a more conservative government took over there. Not long after the end of the negotiations, the DPRK withdrew from the Nuclear Non-Proliferation Treaty and restarted its plutonium-based drive for nuclear weapons, which led to its successful acquisition of such not too long later. This is fish soup that cannot now be turned back into fish or spilled milk that cannot be returned to its container, a disastrous development that we might not now have to be dealing with at this time of leadership transition in North Korea, if Bush and his
Vulcans had not pulled this enormously stupid blunder.