As someone else said, there is no easy answer. One of the reasons I think it's so difficult to answer is that it's not even really clear what the US interests are in the Middle East, and people can have different interpretations. I would identify 1) the security of Israel, 2) energy supply, 3) proliferation of weapons of mass destruction, 4) democracy promotion, 5) terrorism, and 6) the geopolitics of Iran.

I think the Iraq War is a perfect example of the interplay of these interests. The primary rationale offered for the invasion of Iraq was that Saddam Hussein had active biological, chemical, and nuclear programs, but that turned out to be false. The Bush administration later put a lot more emphasis on democracy promotion as its core principle, which was laid out in President Bush's 2nd inaugural address. However, the facts that Saddam's Iraq was a major oil producing country and very hostile to Israel are hard to ignore when looking at the war in a broader sense. The tragic irony of the Iraq War, and the reason that it was opposed by prominent realist scholars, is that with regard to US national security and regional geopolitics, this war was a huge blunder. The US is now living with the results of a failed policy - the fall of Saddam Hussein and the twin rise of ISIS and Iran.

Now if you take the issue of the Syrian civil war as another example, and the Arab Spring more generally, the US stance is predicated on democracy promotion. I believe, and this is my interpretation, that the State Department in President Obama's first term, under Hillary Clinton, saw the Arab political awakening as an opportunity to rehabilitate the US image among the "Arab street", to improve ties with the Arab world, and in the long-run undermine Islamic radicalism by empowering Arab liberals and moderate Islamists. It was an extension of and a twist on the Freedom Agenda. But the Syrian civil war is complex. We already saw in Iraq (and in Libya) that the interest of democracy promotion runs counter to the interest of US national security (proliferation of radical ideology and terrorist groups). Nowhere is that more true than in Syria. Syria, being the main Arab ally of Iran, is also a major geopolitical flash point, and the contest is colored in sectarian terms as well with Saudi Arabia and other Sunni countries seeking challenging the Iranians. However, the United States and European countries, as well as Russia and China, are in the process of (attempting to) reach a diplomatic agreement with Iran on its nuclear program and seem to be testing the waters for more diplomatic and economic engagement. So we see that terrorism and geopolitics, where US interests took a hit in Iraq (although the geopolitical issue is the exact opposite in this case), might suggest that the US not take a decisively hard stance against the Syrian government. But when the chemical weapons issue was raised, it complicated things. The fact that the Obama administration was close to bombing the Syrian government over WMD and then backtracked, and that the US is now bombing ISIS, shows the administration being pulled in two different directions, but it's pretty revealing about their priorities. And finally, Israel's position on the Iranian nuclear program is much more hard line than the West's and they feel that a diplomatic agreement could compromise their security. So you see, all these interests come into conflict.

I don't know if I answered the question directly, but those are some of my thoughts.