WI: Flight 93 makes it to Washington, D.C.

(1) Scenario where 1st NYC plane hits but 2nd is taken down by passengers/fighter jet would have lots fewer casualties. Remember, the first tower hit was the second to fall - the plane hit one of the faces square and very high, while the second tower was hit lower and at a corner, resulting in greater stress to the structure. Burning jet fuel lowered the structural strength of the steel; the lower hit had more strain and thus failed first. The second tower took quite some time more to collapse (IIRC another 30 minutes) and the small earthquake-intensity shaking of the second tower collapsing probably accelerated the fall of the first one (by shaking loose broken trusses, for example, increasing stress on the remaining structure). There's an outside possibility that the 1st tower does not collapse at all, although the damage is probably enough that it will be demolished rather than fixed. More likely, the first tower lingers for longer, allowing more people to escape and a full evacuation of the second tower, which began evacuation right after the first one was hit. Casualties are much lower overall. Second tower is severely damaged but structurally sound after all is said and done, and gets fixed up. Insulation/trusses pointed as factors in the rapid collapse of the first tower as OTL and are replaced/beefed up while repairs go on.
 
(all planes hit, GWB killed)

Cheney becomes president, and the unity mood probably gets a hawkish Democrat (Lieberman or Joe Biden most likely at this juncture) in the VP slot. Cheney really is harsher than Bush, and the nation probably isn't in a mood for nonsense: the scenario that only narrowly got shot down OTL is instead implemented.

It is not well known, but several allied and neutral powers (Russia, France, etc) greenlighted limited nuclear retaliation. Britain was not in favor, and Bush valued Blair's opinion enough to deepsix the idea. Here, the plan goes forward, with Al-Qaeda members getting a thermonuclear tan. Afghanistan is invaded and occupied as OTL, but the Iraq war doesn't happen - Saddam does what Ghadaffi did OTL and lets the US look at/investigate/whatever to its heart's content, rather than risk pissing off an America with a proven willingness to hit the button.
 
More likely, the first tower lingers for longer, allowing more people to escape and a full evacuation of the second tower, which began evacuation right after the first one was hit.
IIRC Authorities in the second tower advised every one to stay in place, as there was no danger to the second tower.
 
While they did say this, many people evacuated anyways (or finished up various things in the office and then left, rather than simply bugging out). In addition to people evacuating on their own, it's likely they order everyone out after the Pentagon is hit.
 
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