Trump was literally impeached for secretly withholding congressionally appropriated military aid to Ukraine and then extorting the Ukrainian president to make up a fake scandal about his political opponent in return for keeping aid flowing. Several of Trump’s top advisors/officials were directly corruptly involved with previous Russian puppet leaders of Ukraine and corrupt pro-Russia Ukrainian oligarchs, notably Paul Manafort, who had directly worked for years to undermine Ukrainian democracy (Manafort was imprisoned in the USA for his illegal Ukraine-related fraud, among other charges. If you want to single out an American to blame for the current war, Manafort is your best bet.)

Remember, Trump fired the US ambassador to Ukraine because she was opposed to Ukrainian (and American) corruption, and instead sent Rudy Giuliani, Rick Perry, and Gordon Sondland – an incompetent hotel developer who bought his way into a cosplay job as EU Ambassador – to further his corrupt schemes, including not only the extortion plan to invent scandals about Biden, but also replacing the leadership of a Ukrainian state-owned gas company so that it could give contracts to Trump’s corrupt American allies, ending the US attempts to extradite Firtash (a criminal Ukrainian oligarch with Russian mafia ties who hired Rudy Giuliani and other Trump allies), etc.

When Trump’s extortion scheme was exposed and blew up as a scandal, Congress appropriated additional aid for Ukraine, and Trump was driven by the scandal and intense public and congressional scrutiny (not to mention pressure from the US military) to continue the aid. (He still wanted to be re-elected, remember, so didn’t want to look totally corrupt.)

Thankfully the USA is not (as of 2016–2022) a dictatorship, and the president does not personally decide all national policy. So when Trump had some bright ideas like nuking a hurricane or selling off Puerto Rico, those didn’t actually happen. When Trump suggested pulling the US out of NATO, there were still enough powerful and sane figures in the US defense establishment who were able to thwart that proposal.

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You are right that the US and NATO did some (though in retrospect it perhaps should have been more) to help Ukraine from 2014–2021. That training, funding, and support was surely one important part of the Ukrainian army’s recent success. Score one for the US military and US federal government.

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> who say that Merkel was always against German dependence on Russian gas

I don’t know about Merkel per se but German political parties clearly have significant corruption problems. The German response to the Ukraine war has been a disappointing indictment of the whole German political class.

More generally, many Europeans (and Americans, etc.) have been bought off (explicitly or implicitly) by money embezzled from the Russian people by various oligarchs and organized criminals. Russian corruption is a serious worldwide problem, and other countries should continue to make large-scale coordinated responses, ending money laundering, seizing corrupt assets, and imposing legal liability on local residents (e.g. the legal, real estate, and financial services industries) who knowingly participate.