I used to be a libertarian until I realized I couldn't stand other libertarians and didn't want to waste my vote any more. I think the libertarian reply to 9/11 (it's our fault! Let's become more isolationist and they'll leave us alone!) finally turned me off the party forever. At least Republicans and Democrats are willing to support a muscular foreign policy. We get about as many wars under Democrats as we do under Republicans, and we would have ended up invading Afghanistan and Iraq no matter who was President. I realized I couldn't deal with this complete denial of America's role in the world, that mainstream libertarians were saying we need to withdraw in on ourselves and try not to be noticed.
As if our foreign policy had no fault. I think the Libertarian position is a reaction against the sentiment that America should be able to stomp all over the world to satiate its desires without any consideration for other nations or any thought for consequences. A muscular foreign policy needs to be balanced by lots of debate and at least some restraint. I assume that you actually argue that America does have an important and unique role in the world, and while I agree with that, our foreign policy has often been based on immoral or unwise considerations. Of course many of those considerations seemed wise, and even moral, at the time, but part of moving forward is acknowledging and correcting, when possible, the mistakes of the past, well-intentioned or not.
While most people agreed that invading Afghanistan was the correct thing to do (The Nation's editorial board, I think, didn't), it's not clear at all that Iraq would have been invaded regardless of who is President. I know Kerry supported the potential for an invasion of Iraq, but it's pretty clear that he at least disagreed with the timing. Who knows what would have happend had we waited. I certianly don't. And Dean clearly wouldn't have invaded Iraq, and Gore might not have either. Republican Pat Buchanan wouldn't have, though I do realize he's not much of a mainstream republican.
I agree that Democrats are just as likely to start wars (just and unjust, if you care to make that distinction) as are Republicans. For example: Vietnam. I seriously doubt Iraq will ever equal that mess of a police action. Also, FDR and Truman destroyed hundreds of thousands of civilians in WWII. So I'm not arguing specifically for or against either party. Besides, parties change, often drastically, over time.
And I also agree that isolationism is dangerous and even wrong, especially in light of the mess we and other western nations have left some of the world in, but certainly unrestrained imperialism is more dangerous. I agree that America is a great nation with unique responsibilities, and has done much good, mostly because of its wealth, and I disagree with isolationism. But I think our past foreign policy decisions, some dating back to WWI, have some blame in today's mess.
To put all of the cards on the table, I'm a registered Democrat, and I agree, to a certain point, with the Libertarian Party's social policies on drugs, sexual rights and capital punishment (at least I think the party is opposed to the death penalty), but disagree with its foreign and economic policies.