Kuwait had no treaty obligation with the US.
OK, a short history lesson. Back in the 1500s-1600s in Europe, there was a dreadful war called The Hundred Year War, which caused tremendous disruption and massive loss of life.
A the end, all the great European powers signed something called the Treaty of Westphalia. The fundamental principle to this treaty is sovereignty. In other words, you can do what you like in your country, and no one outside your country can do anything about it.
Respect for sovereignty is the fundamental principle of the UN, too. Many countries' leaders are tyrants and torturers, yet they can all sit at the UN and speak on behalf of their country.
In WW2, when Germany invaded Poland, Britain and France declared war on Germany because of the lack of respect for Westphalian principles by Germany. They wanted to preserve Polnd's sovereignty.
When Kuwait was invaded by Iraq, the UN allowed the allies to push Iraq out of Kuwait, to restore Kuwaiti sovereignty. The Kuwaiti government are not nice people (one sheik said why fight myself when I have my white slaves from America to do this for me), but the idea is that their sovereign government should be preserved. The Allied colation was not allowed to go into Irq, because it was entitled to preserve its sovereignty, too.
Unfortunately as a way of putting pressure on Saddam, Bush 1 called on the Iraqi people to revolt against Saddam, but didn't back them up with troops because of the lack of UN mandate. Many Iraqi military units switched sides, but without allied backing were wiped out by Saddam's loyal Republican Guard. (See the excellent George Clooney / Mark Wahlberg movie Three Kings for more on this). Same goes for the Kurds in the north, and the Shi'ite Muslims in the south. To stop their annihilation by helicopter gunship, the allies did impose a no-fly zone, but did not take any further steps.
One of the complaints about the current US/UK plans against Iraq is that it breaches Westphalian principles. Iraq has not done anything, and an invasion on a pre-emptive basis undermines the sovereign system of government and international affairs which has been in place since the 1600s.
Personally I think Westphalian principles are flawed, a view I recently found is shared by Kofi Annan in so far as it comes to cries against humanity. Other than Iraq, though, the US is a big supporter of Westphalian principles, refusing to compromise its sovereignty on stuff like the International Criminal Court (Us soldiers on trial for war crimes by foreign judges doesn't appeal to the US).
That's the long and the short of it.
