Not so long ago--a little over a decade, in fact--US military intervention was viewed very differently than it's seen today. The US had been the "good guys" for almost a century, beginning with critical roles in winning both the first and second World Wars.
We made missteps, sure. Dirty as some of them were, the proxy wars during the Cold War could be rationalized (at least in the US, and perhaps by many allies) as part of standing against the perceived threat of communism. Vietnam certainly couldn't be called a victory, but Korea wasn't a defeat. The Cold War died with a whimper rather than a bang that could have scoured human life from the Earth. Over the course of 40+ years, we showed that we were a rational country that could and did act with restraint.
Bosnia and the first Iraq war were both powerful examples of how effectively the US could assemble a coalition for good. Somalia worked out poorly and left a non-trivial fraction of Americans feeling snake-bit, but the country had good intentions and humanitarian goals in becoming involved.
All of that, as we know, was before September 11th.
Before the US engaged in two wars of choice--one to "get the bad guy", and the other to "get the guy my daddy should have got." Before the US government attempted to con the United Nations into supporting the invasion of Iraq. Before the American populace watched almost 7,000 citizen-soldiers die, with more than 50,000 wounded--most of them fighting a war in a country that had no connection whatsoever to September 11th. Before we and the world were shown time and time again that America's belief in the sanctity of basic human rights only applied to American humans. Before a high-school dropout let the world know he had access to some of the most powerful tools available to the US intelligence community, and could use them on anyone, anywhere, on a whim. Before our allies were publicly notified that the US can and does spy on them as a matter of course.
I'll be amazed if any nation in the world follows America's lead into a war nowadays. Some of our traditional allies were burned by supporting the invasion of Iraq, others' reticence has been retroactively justified many times over. Our country's treatment of prisoners in Afghanistan and at Guantanamo Bay has been poor enough that we can't ignore the possibility that they'll become terrorists due to their treatment.
Suppose we could hit the rewind button. Suppose we imagine a world where the September 11th attacks happened, but that the US reacted in the rational, reasonable manner that kept bombs from dropping during the Cold War. Suppose the US invaded Afghanistan and actually committed the resources needed to catch Bin Laden at Tora Bora.
Suppose that the US never pursued a vendetta against Saddam Hussein.
Where might we be?
Certainly, almost 7,000 fewer dead American citizen-soldiers, and about 50,000 fewer wounded. Hundreds of billions of dollars subtracted from our national debt and annual deficit.
Most likely, a national willingness to continue the humanitarian interventions of the Clinton era--including direct intervention during the Arab Spring, of which the civil war in Syria is the latest chapter.
Would we have become involved in Syria before the latest chemical weapons attacks? Not necessarily, but you could definitely justify answering "yes". The US pulled NATO into Bosnia without either side of that war using chemical weapons, intervened in Somalia when neither side was using chemical weapons, and assembled a coalition to push Iraq out of Kuwait without anyone involved using chemical weapons.
September 11th changed everything...but not in the way most folks thought at the time.