I am a little confused about how isolationist the USA were before World War II. Did the USA really back out of every European conflict or did they try to meddle in some way (like an almost World Police)?
The notion of an isolationist U.S. is a myth. The absence of large scale U.S. intervention in European affairs prior to 1898 was due to a combination of limited capability and the need to consolidate on the North American continent, rather than a sense that the U.S. should absent itself from the world stage. In spite of this the U.S. did enter into numerous treaties with European powers, most notably Jay's Treaty of 1794 which normalized trade relations with Britain despite their ongoing war with France. The first military intervention occurred in 1801 as the U.S. engaged in the Barbary Wars in North Africa. For most of the first half of the 19th century the U.S. role in Europe was as exporter of raw goods and occasionally expressing sympathy for democratic movements, such as in Greece in the 1820s.
The U.S. was exceptionally interventionist from its earliest days within North America, using manufactured crises in Florida and Mississippi to claim territories from European powers, taking advantage of Napoleon's need for currency in the Louisiana Purchase and more blatantly fighting Mexico over Texas and taking vast swaths of territory upon victory. Over the second half of the 19th century the U.S. increasingly projected power into Central and South America. Upon the "closing of the frontier" in the late 19th century U.S. policy makers increasingly looked abroad and became involved in Hawaii, Samoa, Guam and fought Spain over Cuba, taking Puerto Rico and the Philippines in the process.
Historians on both sides of the political spectrum are in agreement that the United States established an ideology of intervention early on. Robert Kagan argues this served the world well in Dangerous Nation and Michael Hunt takes a more jaded view in Ideology and U.S. Foreign policy
TL;DR The U.S. didn't engage with Europe not because of sense of isolation but rather because it was establishing hegemony in the western hemisphere.
You can start with the beginning of the US.
From George Washington's Farewell address.
"The great rule of conduct for us, in regard to foreign nations, is, in extending our commercial relations, to have with them as little political connexion as possible. So far as we have already formed engagements, let them be fulfilled with perfect good faith. Here let us stop.
Europe has a set of primary interests, which to us have none, or a very remote relation. Hence she must be engaged in frequent controversies, the causes of which are essentially foreign to our concerns. Hence, therefore, it must be unwise in us to implicate ourselves, by artificial ties, in the ordinary vicissitudes of her politics, or the ordinary combinations and collisions of her friendships or enmities."
Thomas Jefferson (and is famously misquoted as being Washington) saying in his inaugural address:
"peace, commerce, and honest friendship with all nations, entangling alliances with none."
The US did not get involved(Outside of commercial interests) with Europe till arguably the Spanish American War. But After that it was fairly active politically.
Let's see off the top of my head:
Outside of the two wars the rest where Military and or political interventions into European affairs.
However even during world war one Woodrow Wilson ran on the slogan "He Kept Us Out of the War". But this became untenable after the Zimmerman Affair and the rest is History.
http://en.wikisource.org/wiki/Washington%27s_Farewell_Address
http://en.wikipedia.org/wiki/Timeline_of_United_States_military_operations
http://en.wikipedia.org/wiki/Washington_Naval_Treaty
http://en.wikipedia.org/wiki/Zimmermann_Telegram
http://en.wikipedia.org/wiki/United_States_non-interventionism
/u/cubshound answered the question really well, but to hammer his point home, the word isolationism doesn't even appear in American dictionaries until the end of the 19th century, the entire concept of blindly avoiding all foreign involvement (especially European involvement) was reflected back on early American history by later people. (Their was a period of popular sentiment supporting explicit isolationism in between the two World Wars, but thats a separate phenomena.)
So why was the US so uninvolved in European affairs if they weren't isolationist? Well, the 19th century was actually pretty peaceful after Napoleon's defeat (and the US did have a quasi-war with France during that period so the US was already involved in European politics at the time. The only large major conflict in Europe prior to the later Prussian wars was the Crimean War, which the US almost become involved in (though nobody really talks about this anymore because the politics was overshadowed by domestic concerns and the Civil War.)
In short, isolationism as a historic US policy is a myth.