Why

Why did USA
become from a semi-isolationist conservative-liberal country into some kind of World Police? When did it start?

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It started in the aftermath of WW2.

And as for why, it was primarily because the U.S. had commercial interests all over the world, and it turned out that isolationism wasn't doing a good job of safeguarding them. Without U.S. presence and threat of force keeping things open, the world was a lot less stable, and to this day, you pretty much only see open, conventional conflict in places that the U.S. isn't willing to bludgeon people into cutting it out.

Most countries who are in a conflict is because of the US though. If you wouldnt invade countries to get more oil and money the world would be a more peacefull place

after WW2 US became a majority power to balance Soviet and comunist hegemony power

That's why in cold war, when a civil war happen there are always Us or communist power that backed a side

Well the United States primary influence isn't through the direct invasions. It is through using economic power to support allies. These allies are often awful people who harm their counties though, but they support American policies.
And I do think the American Military as a whole acts a stabilizing force. Since essentially it is an implicit threat against any small to upper mid sized national that breaks the United States rules.

America was on the world stage as early as the Tunisian War

...

America reached an unreal level of industrial power in the 1800s. By the time of the first world war, America was a juggernaut and no matter how much euros might hate america, theres no denying that either side would benefit a lot from having America as an ally.

By the time of the second world war, America was even bigger and better. Before they even entered the war, they were essentially the sugar daddy of the allies, supplying the soviets and the British and the other powers. And after the war, all of Europe was in ruin while America was essentially untouched by the war. The three big dogs of Europe, Germany, France, and UK were all shells of their former selves, and the Soviet Union, despite being battered to shit, was still the most powerful place and the US was the only one who could check them.

It was to the point where the Marshall Plan happened. The US was rebuilding Europe AND protecting them from the Russians. Europeans didnt realize this, and Britain and France both got cucked by the US during the Suez Crises.

If you think that multiple similarly strong forces would lead to more stability than the single superpower you're very wrong.

You are pain release.
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