Well, we should have partitioned Iraq. Why do we need to respect borders the British made up decades ago, when they just won’t work. That’s on Bush as well as Obama. And we needed to let Maliki know he had something to lose if he pushed the Sunnis so hard war was inevitable.
The partition of India involved sectarian massacres and a year-long border war. Partitioning Iraq and leaving sounds like a panacea, but it would simply have kicked off today's conflict a decade earlier. Bush wanted to appear a statesman rather than a Vandal or an anarchist. My guess is that the same brain trust that had him appoint Karzai and Maliki would also have given him a hard time if he had simply partitioned Iraq and left.
It's only in hindsight, after 8 years of troubles in Iraq, that partition seems like a good idea. At the time, the country seemed like it might hold together without too much work on our part. The politically-incorrect truth is that we simply did not kill enough of Iraq's combat-age men during major combat operations. We needed to kill the whole snake rather than merely cut off its head. When Germany and Japan surrendered, they were all fought out. Between 1/8 to 1/4 of their (18-28) young men were dead. Many of the rest were shell-shocked or wounded. That, rather than any epiphany, was why we had so little postwar trouble with them. They had had enough. Major combat ops in Iraq were over in the blink of an eye. What we saved on the front end, we paid on the back end, and then some.