Can someone explain to me why the afghan war is unjust?

[Tonus]

We didn't have an exit strategy because there was no real exit strategy that could ever be formulated.

I don't think that there was ever an exit strategy. The middle east is a socio-political powder keg sitting on a very valuable resource. That resource confers power to those who control it, and so people will fight to control it. (I read recently that the USA is now the world's biggest producer of oil, which is one reason that OPEC has not limited production and allowed gas prices to tank. Imagine if we'd known this in 2003?)

A map of the middle east shows that Afghanistan shares a border with both Iran and Pakistan. Having some level of influence in Afghanistan lets the USA have some influence on those nations; having military bases and personnel there lets the USA potentially have a LOT of influence. Same with Iraq, except that it shares borders with Iran (again), Syria, Turkey, Jordan, and Saudi Arabia. Iraq pretty much sits right in the middle of the middle east.

I think the plan was to make those two nations "friendly" and have them allow a significant US military presence, which would grant enormous political power in the region. The plan went badly because the odds of the plan going badly were simply that high. Getting friendly governments installed via 'democratic' means would have been extremely difficult even without the widespread religious fundamentalist guerrilla army with deep ideological ties to the people (and the visitors with their deep religious divide with those same people).

It's one of the reasons I think we should bring nearly all of our troops home and concentrate on defending our shores and leaving everyone else to their own devices. I don't think it's realistic, but it would prevent this kind of pie-in-the-sky planning that will nearly always end badly, and nearly always end badly in ways we didn't imagine.