Ruth Marcus, I think, largely has this right: "In short, Bush inherited a budget in healthy shape. He left it in tatters. The faltering economy played a supporting role, but the chief factors were of Bush's making: his tax cuts, his wars, his prescription drug bill. Without these, the country would have been running surpluses during his tenure. The wars will wind down, but the price of the tax cuts and prescription drug bill will climb even higher over the next decade."
The only thing I'd say in even mild defense of Bush is to suggest that the formulation "his wars" is only half right. Iraq was a terrible blunder, but the decision to go to war in Afghanistan was a rational reaction to the 9/11 attack; there's something off-note about calling it his war when Americans were all on board, and in fact probably would have chased from office any president who didn't respond to the attack in similar fashion. Marcus is right, though, that Bush is the first president to put a war on the credit card. His attempts to evade responsibility for the country's budget problems is very far from convincing.