Why is the United States considered a Western country?
USA and Canada and Australia and New Zealand are all considered western nations and part of the Anglo world with U.K. Because of their cluture, policy, politics, belief, system, and ways of life.
Sure the USA doesn't agree with other western countries in some aspect and there is a difference between them but this can be applied to anyother countries. Western civilization doesn't mean EXACT civilization, but similar, and democracy, freedom, and rights and cluture is what keeps them together. But most importantly the way developed nations act in fundamental part of being western. In fact, I'D argue S.K. And Japan are/should be considered Western after WW2 and Korean War.
USA is to the world what U.K. Is to Europe, they are the "out of placed" nation, USA is way more conservative than U.K. But U.K. Is more conservative than Germany and Western Europe and Canada is same as Germany.
USA has representative constitution republic, Anglo countries like U.K. And Canada have Constitutional Monarchy, other western countries have monarchs and democratic principles and others have direct or electoral voting. All are different but they are all Democracy.
They may have trade with one country and not with other, but they all believe in open market free trade and all.
ideology, identity, history, development, ideals, aspects of life, and cluture makes America a western country and in fact many look at as the bedrock of the ideology. Having disagreements about policy means nothing as no 2 country can live on earth without dissatisfaction with each other.
If you look at it in purely geographic terms, the West starts at Istanbul and goes West for 180 degrees. So, it includes Western Europe through the Americas. It would even include Africa, which makes some sense, but not enough. I would call the West. Europe, the Americas, Australia and New Zealand. They share a history, languages and a set of cultures that are related.
If you look at Western food, compared to that a Japan, China and S. Korea, it is easy to put Europe and the Americas on one side and the Asians on another. The Indian subcontinent is a bit of an outlier here. It's food is far more similar to Western than it is to East Asian. India did not share Western Industrial development. So, even if it is quite different from East Asia, it doesn't belong with the West.
[edited to add]
I purposely didn't mention Russia, but I would call the Western parts of Russia as Western and anything in Eastern Europe. Their history is European history. Their culture is European, though most Russians probably feel like step-children of the West.
Well, it is controlled by European-descended people, has "Western values", has more recently warred with Europe than against it, and as for your question details, if the standard for being a Western country is "never opposed another Western country", England, France, and Germany are out. It's a round globe, after all, and although Australia is no more Western than Japan, it's considered a Western country. "The West" is a theory and an idea, not a place.
It's more a matter of allegiances and culture that geographical location, although North Maerica IS west of Europe and Europe was considered the "center" by Europeans for a very long time.
Would you consider Australia and New Zealand as Eastern or Western countries?
US leads Europe and is a member of NATO and so many other treaties.
US is also a leader of free world, it cannot separate from Europe. Countries like Australia New Zealand, Canada, Japan South Korea, and Singapore all looks for US leadership and also they look for defence of their country.