Watching Gen. David Petraeus testify before Congress inspired several thoughts.

The first is that he has taken on one of the most difficult missions ever given a U.S. commander: building a nation in a region of the Middle East already involved in an incipient ethno-sectarian war.

Petraeus unambiguously identified this as the core struggle we face. "The fundamental source of the conflict in Iraq is competition among ethnic and sectarian communities for power and resources," he said. "The question is whether the competition takes place more -- or less -- violently."

The second thought is that it would be difficult to find a man better suited to this monumental challenge. Petraeus is very smart, honest and tough.

The third is that the courageous troops Petraeus leads are performing as well as military forces can be expected to perform in such a situation.

At one point, Petraeus gave an example of the problems they face. Shiite warlord Muqtada al-Sadr has decreed that his Mahdi Army should take a six-month hiatus from fighting. Some have apparently obeyed his decree -- others have not. As consequence, Petraeus has decided to treat some Mahdi Army members as enemies, some as potential friends.

"We are not going to kill our way out of all these problems in Iraq," he explained. "You're not going to kill or capture all of the Sadr militia any more than we are going to kill or capture all the insurgents in Iraq. And, in fact, what we have tried very hard to do is to identify who the irreconcilables are, if you will, on either end of the spectrum, Sunni and Shia, and then to figure out where do the reconcilables begin and try to reach out to the reconcilables."

"Some of that will have to be done with members of the Jaish al-Mahdi, with Sadr's militia," he concluded. "The question is: Who are the irreconcilables?"

To put this in perspective, recall that we went into Iraq because all of our intelligence agencies with all of their resources could not accurately determine whether Saddam Hussein had weapons of mass destruction. Now, to get out of Iraq, our soldiers must accurately determine which members of a heretofore murderous, Iranian-armed, Shiite fundamentalist militia can be trusted to make peace and which ones cannot.

Yet, in the face of such challenges, our troops have achieved measurable success. "The military objectives of the surge are, in large measure, being met," Petraeus said. Continued...