The Iraq War is easily the biggest: More Americans died fighting it than were killed in 9/11. It contributed to thousands of amputee soldiers and countless PTSD cases. Tens of thousands were killed abroad. It may cost U.S. taxpayers $6 trillion. That policy guarantees Bush's status as a president who weakened America. Bush's decision to order the torture of humans, and to stick hundreds (including innocents) in due-process-free detention, is perhaps his next biggest failure.

Domestically, the combination of Bush's tax cuts, his profligate spending and his failure to anticipate or prevent the most significant financial crisis since the Great Depression guaranteed that he left the country in far worse fiscal shape than he found it. And while I don't particularly blame the Bush Administration for failing to foresee an unprecedented terrorist attack that took the whole world by surprise, the 9/11 Commission report makes clear that it could have been prevented. Let's call that the most major failure for which we can't quite fault Team Bush.

Obama's most significant failures are very different. Nothing he's done has proved as immediately catastrophic as the Iraq War, and he's yet to preside over any event as immediately damaging as the 9/11 attacks or the financial meltdown. Incompetent as he has seemed at times, he has been less so than his predecessor.

His moral failings are arguably comparable. On the theory that it might marginally reduce the risk of a terrorist attack, Bush tortured some and held others in indefinite detention. On the same theory, Obama kills from afar with drones. He's killed thousands of "militants" and hundreds of innocents by most estimates. None of them got any due process. On Obama's watch, the United States has struck wedding convoys and funerals and killed a 16-year-old American citizen. Sometimes his CIA doesn't even know the identity of the people it kills.

It is too early to assess the practical effects of Obama's policies. My fear is that they will prove very costly in the long run: that his approach to mass surveillance makes us alarmingly vulnerable to abuses by a surveillance agency with an undeniable history of willfull lawbreaking; that his whack-a-mole approach to fighting terrorism is distracting us from the fact that he has no long-term counterterrorism strategy; that he is creating more terrorists than are being killed; and that he is squandering the brief moment when America could establish norms surrounding lethal drone strikes before many other countries get them.

We don't know how history will judge this era. Perhaps three decades hence Americans will look back aghast that our leaders did so little about climate change or the overuse of antibiotics or the destructive potential of nanotechnology. What we know now is that the substantive mistakes of Bush and Obama are substantial and different—and that they dwarf any merely political mistakes that seem to show similarities between the two men's missteps.