A good question and one with multiple answers.
The US was, prior to World War One, very much an isolationist country meaning it would keep out of European wars as far as possible. This policy had its origins in the 19th century and in 1812 the US and UK fought a war on US soil, not European. This policy was compounded
somewhat by the ‘Monroe Doctrine’ of 1823 when President James Monroe stated:
The USA will not allow interference in any country of the American continent by any European power, and that any such involvement will be regarded as a danger to the peace and security of the USA itself.
This form of isolationism continued and became more clearly defined in the early 20th century up until the outbreak of World War One. Britain needed the United States in on her side as Kaiser ‘Bill’s’ army proved a more capable fighting force than the British anticipated. There would be no ‘spending Christmas in Berlin 1914’ and the best achieved by the men themselves on both sides (many not really wanting to fight at all) was a cordial game of football and an unofficial ceasefire. The USA refused categorically to become involved in the European slaughter and retrenched into isolationism.
This would be how it remained until the Second World War erupted and Britain once again found herself needing the US involvement against Nazi Germany. President Franklyn D. Roosevelt having been re-elected on a policy of; “our boys will not fight in foreign wars” refused to send US troops to fight on European soil. On 7th December 1941 US isolationism was shattered when the Japanese, an ally of Nazi Germany, launched an unprovoked attack on Pearl Harbour. Despite US efforts to remain neutral, although with sympathetic leanings towards Britain during the war, this forced Roosevelt’s hand. He couldn’t stay out of the conflict, now a world war, any longer.
In 2016 under the strange electoral college rules Donald Trump was elected as President of the United States. He took up office on 20th January 2017 and left office, reluctantly, on 20th January 2021. Trump's election motto was, and is, ‘America First’, and he has shown support for far-right parties around the globe including the fascist ‘Britain First’ who reportedly have links to ‘Ulster’ loyalists. He indicted in his first term his contempt for countries who did “not pay their way towards NATO”. This cavalier nutter of a President has also indicated his tepid support for the Atlantic Treaty and even, if others don’t come up to the mark in contributions, to pull out of NATO.
It is unlikely the USA under President Trump will head back into isolationism, at least not in its traditional sense. What is apparent is the US are changing their allegiance’s possibly away from Europe and NATO to a point and more towards Rusia in pursuit of the mineral deposits beneath the soil of Ukraine! This analysis may in the long term be proved not to be the case but present evidence, like the humiliating of Ukraine’s President Zelenskyy in the Oval Office, suggests this could be the new direction of US foreign policy. As opposed to old style US isolationism perhaps a new kind of US imperialism is the more likely? Only time will tell. Whatever the outcome the present policies of this President are not healthy for the rest of us.