Waaaay off topic, but...
I think that a war against the Taliban was justified, even a bit late, as they were a far more oppressive regime than that in Iraq (which was quite progressive in many ways, compared to Middle-East countries considered "allies" of the U.S) - possibly the worst in the world. And it was largely due to foreign intervention that it got that way.
And it wasn't a particularly Republican war - the original plans were drawn up under Clinton.
Initially, the target was bin Laden and the al Qaida-controlled areas of Afghanistan, and the plan was to launch an offensive from Pakistan, with Pakistani troops and U.S support (satellite photos, reconaissance, equipment). Plans were made and troops were ready by October 1999, but this was interrupted by a military coup in Pakistan on October 12.
The Pakistani military has been a long-standing supporter of the Taliban (still is, largely), and this may have been one of the factors that led to the coup. General Musharraf, who now runs the country, has since been bribed into becoming a U.S ally, but much of the military still objects.
At the time, the Taliban were trying to put on a good face for the U.S, and negotiations were going on to allow an oil pipeline to be run through the country to get oil from Russia out of the region, but missile attacks on the al Quaida camps led them to break off the talks.
The two issues - stopping al Qaida and building an oil pipeline - came together and plans were made to invade Afghanistan and put into place a more friendly regime. The plan was to build an alliance of the various opposition factions in Afghanistan (this was completed with CIA help - they became the "Northern Alliance" you heard about in news reports), and mount an invasion with the help of Russia (which needed the pipeline to sell its oil), as well as India and Pakistan if possible (a "coalition of the willing" - type deal).
I don't know if U.N approval would have been sought, but it became moot after the September 11, 2001 hijackings were viewed as an act of war, and NATO invoked the military support provisions. Support of Russia and others was no longer needed politically, and the war proceeded as planned (this is why it needed only a month of preparation, whereas the war in Iraq needed a year and a half of work).
Here's an article that describes some of it (a web search should give you some other interesting articles):
A parallel might be the Khmer Rouge regime in Cambodia, which was, but many accounts, even more psychopathically repressive than the Taliban. Vietnam eventually intervened due to border attacks from Cambodia and a flood of refugees fleeing the regime, and invaded, overthrowing the regime and leading to elected governments - somewhat ironic as the Khmer Rouge gained power partially due to U.S bombings of the country (war crimes ordered by Henry Kissinger - also an act of war not approved by Congress, and thus also a violation of the U.S constitution. Kissinger has never been tried or punished for either violation).
History is filled with ironies, actually. And not-so-secret "secrets".