First, what is a mission? It has to be a single attack/rescue/defensive effort, whatever. It cannot be a repeated effort--that would be missions, not mission. Thus, a campaign is not a mission. A campaign is a combination of missions. I am trying to think about D-Day--was that a mission or not? Well, it was a focused effort at a single discrete objective--land tens of thousands of troops on Normandy's beaches. But it took so much time--not just the one day really but weeks of bombing, and it took days/weeks to get sufficiently inland. Usually, we would consider a piece of that big operation to be a mission, like the Brits seizing a bridge behind the lines.
Second, this is comparative: there are many missions, of which the Osama mission is just one. But how do we evaluate importance? War-turning? That would be a few, but killing Osama did not end the war on terrorism (since a war on a method of war is unlikely to end soon). But a mission can have a big political impact and it can have a big military impact. Remember the time that these guys blew up some big guns so that the British Navy could rescue heaps of troops in the Eastern Med? In the book, it was more than just rescuing thousands of troops but about keeping Turkey neutral. Hmm.
Ok, so what comes to mind? Just US military history for now.
- Dropping the atomic bomb on Hiroshima. That essentially ended the war and started the Cold War. Huge political ramifications even if Japan might have surrendered shortly anyway. Is it a mission? Sure, one plane, one bomb dropped.
- The Doolittle Raid. A small set of planes taking off from a single carrier. Direct military impact? Not much especially given how much ordnance was dropped later. Political impact much larger--both at home as it gave Americans a chance to cheer for something in the dark days after Pearl Harbor AND it provided some impetus for Japan's attack on Midway which did turn the tide.
- Bastogne. Sending the various units to prevent the fall of the crossroads was a huge deal, although Germany was going to lose anyway (thanks to the USSR!). But the dividing lines in Europe might have been a bit further to the west if the Brits, Americans, Canadians, etc. had to re-take the ground lost here.
- Operation Market Garden? Had it succeeded, the war might have ended a bit sooner with the line drawn further to the east? Of course, the mission really had no chance to succeed--what an incredibly optimistic plan.
- World War I? Nope. Not a single mission comes to mind.
- 1898? Damned if I know.
- Civil War: was there a single mission? Perhaps the defense at Gettysburg that preventing the Confederates from winning on day 1 or day 2. Again, I am not a military historian, and harder to think of this as a mission.
- War of 1812? Please.
- Revolutionary War: Hmmm, crossing the Delaware?
- Korea: the Inchon maneuver was more of an operation rather than a mission, eh? Any particular missions in this war?
- Vietnam? Oh, one could suggest the mission that produced My Lai, but that kind of warps the notion of mission. Gulf of Tonkin, I guess. The mission, whatever it involved, helped to justify escalation. Would escalation have happened anyway? Probably.
- Cuban Missile Crisis: the recon flight that spotted the missiles. Damned important mission.
- Which reminds me of the U-2 mission where the plane got shot down, scuttling some US-Soviet diplomacy.
- The Iran hostage rescue mission was damned important. Failure can very important. In this case, it ultimately led to Delta, if I am not mistaken, and certainly helped lead to Goldwater-Nichols which reformed the US Joint Staff and how the US operates its military.
- Greneda? Ha.
- The peacekeeping effort in Lebanon? Well, the suicide attack on the US barracks probably shaped US and terrorist behavior in the region for a few decades (not to mention affecting Jack Reacher quite directly).
- Any part of the first Gulf War? No mission stands out. Not even the Battle of 73 Easting since the US was going to destroy the Iraqi army wherever it was encountered.
- Nothing in the Afghan war nor the second Iraq war stands out, but I could be missing something.
- The Osama raid. Too soon to really understand the impact on those who might and do support AQ. Too secret to figure out what intel was gained.
Feel free to disagree.