On one hand, the shift to the new counterinsurgency strategy and additional force levels brought to bear—together referred to as 'the surge'—were the keys to turning around the situation in Iraq.  On the other hand, the surge of American resolve illustrated by these actions may have been the most important factor.

From Stratfor:

What was important about the surge is that it happened at all. In the fall of 2006, when the Democrats won both houses of Congress, it appeared a unilateral U.S. withdrawal from Iraq was inevitable. If Bush wouldn’t order it, Congress would force it. All of the factions in Iraq, as well as in neighboring states, calculated that the U.S. presence in Iraq would shortly start to decline and in due course disappear. Bush's order to increase U.S. forces stunned all the regional players and forced a fundamental recalculation. The assumption had been that Bush’s hands were tied and that the United States was no longer a factor. What Bush did -- and this was more important than numbers or tactics -- was demonstrate that his hands were not tied and that the United States could not be discounted.

Hold this up to the light with the report showing how negative media reports embolden terrorists and increase violence to see how those who understand and support the war are correct in saying media bias and war opposition undercut the mission, lengthen the conflict, cause more deaths, and chip away at the chances for success.  Finally, the Iraq War is truly starting to resemble Vietnam: enemies fighting a superpower democracy know the path to victory is through the media to the political leaders and voters.

The enemy thought they could throw the US out, and with that reorder the perception of strength and power around the world.  More than simply an addition of troops, the surge beat down that dream yet again, while killing many of its supposed deliverers.  If Obama is elected, the enemy will have a reason to raise violence levels again, knowing Obama would never be able to match Bush's resolve.

Obama will be tested if elected, challenging the gains that have been made.