I fear that with the current situation in Europe culminating in a collapse of the Euro, England will stand alone in Europe and the rest of the European countries will squabble amongst the remains for themselves. Russia and China will see this as an opportunity to expand, and do so, and with America taking a more backseat in the world policing there will be less involvement from America.

And why not, I ask? The Chinese external economy will collapse and they will face social tension at home, either heading in the direction of a utopian "50s-style" golden era (if they preserve a middle class) and becoming self-sufficient, or plunging into internal sociopolitical chaos.

Russia? Well, they have their share of problems already.

I don't see any legitimate major threats to the world of the class of the Roman or British or German empires, frankly, except for America. And a Ron Paul victory would see that come to an end very, very quickly.

Let's hope for it.

You have already seen how Ron Paul has affected the world. He will come in second or third in the rest of the states during the primary elections and won't be president.
The only reason you even have heard of him is ats has a sizeable population of Paul supporters which isn't representative to the amount of support he ACTUALLY has amongst registered voters compared to other candidates.

The Republican Establishment is in a severe panic over the rise of Ron Paul. It is why you see the artificially created "rises" of people like Bachman and Santorum. We'll see what happens now that they are just about out of "anti-Romneys".
Also, the liberal media largely ignored RP until recently. Now, its a full on smear campaign. Amazing how they brushed him off as a kooky guy with lots of integrity and honesty. But now he's suddenly dangerous and lies about things...hmmm. Sounds like establishment folks are more concerned than some on this thread would like to believe.

To answer the OP's question: RP would not be an isolationist. He would work to reverse the last 50+ years of exacting tribute via debt and corporate domination of 3rd world nations. He would stop paying American taxpayer tribute to the banking system and would stop destroying American currency. He would promote self reliance, at home and abroad (Of course, by example rather than through forced compliance).
Through American leadership (by example and via co-operation rather than corporate and military force) The U.S. could maintain its influence on the world. Some of the positive aspects would stick around. The negatives would disappear.
The downside for Europe? You guys maintain your own security.

in the short term, there is bound to be instability. And there are always unintended consequences of any action, positive or negative.

I thought about this.. I can't come up with anything, we'd have to wait and see.