The maximum possible goes way beyond the things mentioned yet.
Just imagine the US would've implied not only Afghanistan, Saudi Arabia and other Islamic nations where the terrorists came from or where they were trained or supported, but also Germany, France , Spain, (Canada?) and maybe some other nations, for negligence in combatting terrorism. An important person of one of those countries (wrongly, afaik) speculating that their secret services knew about the attack but didn't give the US concrete evidence, because they wanted to play some kind of chess on an international level, might be enough for that in the painful aftermath of 9/11.
Than we'd see the war starting in Afghanistan and Saudi-Arabia at the same time, with some surrounding nations being involved pretty quickly (Pakistan, Oman, Yemen...), and a mass mobilization and war economy in the US.
Also, there'd be extreme pressure on the other countries to contribute soldiers or money, to extradite potential terrorists, and to give up nuclear capabilities, WMD's, and so on.
Russia and GB play along - Russia as they see an opportunity to get more aggressive against rebellions in the south, GB because of Tony Blair.
Canada is forced to harmonize immigration procedures, defense, and other activities with the US, leave the commonwealth and declare independence of the UK. Blair won't be happy about it, but I suppose he wouldn't want to make the US angry at that moment.
With shady proofs in their hand, similar to the ones for WMDs in Iraq, the US starts a surprise attack on the not very cooperative Germany under "peace-chancellor" Schroeder. France doesn't like the US as neighbouring country and feels like being the next target, so they side with Germany, trying to help them. But Germany falls nearly as quickly as Serbia did a few years earlier, and after that, France is next. The EU is dissolved, some European countries ally with the US.
As the US does her "minimum civilian losses" policy like in OTL Serbia and Iraq, France doesn't use her nuclear bombs and is quickly won over. After that, the US attacks Iran and the whole rest of the Middle East, except Israel and Turkey.
In a departure from OTLs after-war policy, the US declares Islam to be responsible for the terrorism, and outlaws this religion in every territory under her control. The Christian right in the US celebrates for days. The people get a neat little book in their respective language outlining the different other religions available (not only different variants Christianity, but also Buddhisms, Hinduisms, Confucianisms, Shintoisms, Judaisms, Humanisms, and a few others). They have to choose their new religion or face jail. They also have to prove they study the religion and engage in community activities of that religion. And they have to send their children to the respective religious teachers flown in for that.
To help this process, people are relocated into big cities according to the religion they choose. That has several advantages: The production of bombs and the likes can be controlled more easily. Weapons can be kept away from the people more easily. Fewer imported religious leaders are needed to teach the people their new religions. It's easier to protect the foreigners involved. It's easier to keep sabotage from happening. And so on. Also, cities with the same religious majority in different countries are put close to each other, to encourage break up of old countries and forming of new countries, so that nationalism will play less of a role in rebellions.
Furthermore, the US takes people from conquered countries and employs them in other conquered countries. This way, American lives are saved, terrorists have fewer targets, more people depend on the US, costs are lower, and so on.
With their anti-Islamic policy, the US has nearly all Muslim countries against her. They make the Russians employ the same policy in their sphere of influence, to divide the attention of potential terrorists. For that, the Russians get a pretty much pacified and stinger-free Afghanistan. China also happily joins in the conversion of her Muslim minority, so do many other countries. Some Muslim countries in North Africa and South Asia bow to American pressure to employ the same "reforms" themselves, others are conquered first. Islam slowly ceases to exist. A few pockets of stubborn (partly secretive) Islamism don't pose any threat after some time.
Germany is divided into four or five states, who, together with France, are made independent again.
In the former Islamic world, the countries are also made (pretty) independent as soon as their non-Muslim societies are stable enough to survive on their own.
The states of Canada are made US states against the protest of mainly the UK, but without them being able to do something against it. That also strengthens the republican movement in Australia, so that Australia goes the same way without US pressure.