It seemed straightforward in the days after terrorists smashed jetliners into the World Trade Centre and the Pentagon.
Leaders of NATO countries knew they had to stand by their traumatized ally. For a time, their voting publics understood this too.
But nine years since the invasion of Afghanistan, Americans wonder why more of their blood and treasure should be spent on what seems like a failing venture. As a result, President Barack Obama has scaled back U.S. ambitions for success in Afghanistan and put the world on notice that there is a time limit to the U.S. commitment.
Waning support from Canadians helped prompt the Harper government to agree to a 2011 end to our combat presence.
Britons, skeptical about the shape-shifting rationales for invading Iraq and Afghanistan, have stopped buying the argument that Britain's overseas entanglements are reducing, rather than fuelling, extremism at home.
The Netherlands, which saw a government fall over the mission earlier this year, kept a promise to withdraw all 1,950 Dutch troops in August.
Welcome to the dilemma facing leaders in the 21st century. The conflicts of the future will likely be smaller-scale versions of the messy affairs of the past decade: long, complicated, with goals that are hard to define and harder to achieve.
At some point, Western governments will be urged to act, whether to try to untangle the tribal complexities in Afghanistan, getting between rival factions in Congo or prodding authorities in Yemen, Somalia or the Philippines to root out Muslim radicals plotting attacks on U.S. soil.
The question for leaders after Iraq and Afghanistan is this: Can you sell a war that no one wants? And if you can't,, how does that affect global security?
"The bar is probably higher now," says Eugene Lang, a former adviser to two Liberal defence ministers and co-author, with Janice Gross Stein, of The Unexpected War: Canada in Kandahar.
"After 9/11, the bar was pretty low, because you'd had a really game-changing event, people believed the world had changed fundamentally overnight, and on top of that you had relatively strong economies."
All of the advanced economies face years of high unemployment, tepid real-estate markets and belt-tightening measures that aren't conducive to expensive military adventures, where casualties and strategic missteps are visible to voters.
But those dynamics may be most acute in the U.S., exacerbated by feelings that America's traditional allies are cutting defence budgets to the point of uselessness, and that emerging powers should step up and do more.
"Americans will say, 'Why should it be us?' if it is truly a post-American world, or a post-Western world," says Christopher Coker, a professor of strategic studies and international relations at the London School of Economics.
"They will be looking at the cheaper options more and more, as the politically sustainable ones, and the economically sustainable ones."
That means responses could be "much more minimalist" for the foreseeable future, Prof. Coker said: unmanned drones for things such as targeted assassinations; quick special forces operations on the ground as needed, and no drawn-out attempts at "nation-building under fire" by large numbers of conventional troops.
More than 40 countries, including Canada, are investing in drone technology. The United States alone has quadrupled drone strikes in the tribal areas of Pakistan since Mr. Obama was elected, and the military plans to double production of them next year.
"They can endure strategically, and so can the country, because the country isn't asked to make any sacrifices," Prof. Coker said.
The irony, however, is that by heeding the lack of public appetite for a heavy ground presence - which many experts still insist offers the best hope of keeping local populations onside - politicians might end up making things worse.
Drone strikes in the tribal areas of Pakistan, for example, appear to have done little in the battle for hearts and minds. It's unclear how many civilians the drones are killing, or whether new recruits are being driven into the arms of the Taliban as a result of the thunderous Hellfire missiles crashing around them.
But what is clear is that the relentless drone campaign is creating an atmosphere of fear and paranoia among citizens.
A recent poll by the New America Foundation, a Washington think tank, of residents in the tribal areas of western Pakistan found 38 per cent feel the United States is the biggest threat to their personal safety, compared with 16 per cent who said it was the Taliban. The study also found 76 per cent oppose the drone attacks, and 77 per cent believe the U.S.-led war on terror's real purpose is to weaken and divide the Muslim world to ensure American domination.
Is it possible that a decade of favouring quick fixes such as drones to appease a generation of war-weary Western voters could burden the next generation with costlier conflicts to undo the damage?
"Minimal strategies are affordable," Prof. Coker says. "Whether they're effective or not is a completely different matter, but they're the ones that will be most attractive, at least in the short to medium term."
Stephen Saideman, a political science professor at McGill University who specializes in international security, questions whether drones are necessarily the way of the future.
"Drones have utility, but they can only work if we have people on the ground gathering intelligence."
After all, if the real ticket out of Afghanistan and other failed states is to train local armies so they can fight insurgencies themselves, that requires years of side-by-side nurturing and teaching. That means boots on the ground - not just drones in the air.
"The standard language is they can fight their wars better than we can," Prof. Saideman says. "But they can't get there until we train them. That involves infantry, special forces, guys on the ground with guns."
But short-term pain for long-term gain no longer seems an acceptable argument, for voters or for the governments they elect - and turf out.
"It's going to be really hard to do anything like Afghanistan again in the future, because all these countries are stretched, and burned by it," Prof. Saideman argues. "Politicians paid a lot of cost for it domestically, and we still don't know where it's all going."
Nevertheless, he predicts voters could still be galvanized to support interventions in the case of "a humanitarian crisis that is so clearly one-sided that the publics of NATO countries become so motivated to say, 'Afghanistan was hard, but we're going to do this because what we see here in country X is so awful.' "
Indeed, though polls show Canadian politicians could face voters' wrath if they authorized another overseas mission in the next while, some observers say our self-image as an outward-looking, benevolent force for good in the world will ultimately prevail.
With so many foreign-born Canadians, internationalism is increasingly becoming "part of our DNA," or our "new nationalism," says Norman Hillmer, a Carleton University professor and leading authority on Canadian military and political history.
Plus, starting in the late 1990s and through the Afghanistan mission, he notes, Canadians have become more at ease with and "more respectful" of our military past, and less attached to the idea that we should be peacekeepers.
None of that means selling future wars to Canadians will be easy, with today's battles fought in a 24-hour, real-time media environment where "every death and every setback gets sensationalized," Prof. Hillmer says.