The American retreat largely stems from the election of Donald Trump as the president of the US who has strikingly different world view than his predecessors. He envisions the US as a leading power in the international system without wanting to take the responsibilities that come with being a leading power in the global order. He does not want to spend any American resources unless they very narrowly and very specifically meet American needs as he defines them.
During Obama’s presidency, there was a shrink in defence budgets, but that did not mean a retreat from global engagement, unlike Trump. The current POTUS shows a complete lack of interest in supporting multilateral organisations and multilateral treaties and a dramatic increase in the defence budget exceeding $700 billion.