Alone? Maybe.

Vulnerable and weak?

Um, anyone who believes this is just not engaging in dispassionate analysis of the situation. We are a lot of not so good things. Weak is not one of them.

Any nation alone in a world of nations with strong alliances is weak.

The US isn’t alone though. Again, dispassionate analysis.

It is increasingly alone, by its own choice.

Exactly. Right wingers fundamentally misunderstand the source of American power. China might have a bigger navy that the US, very soon. Russia has more nukes. But those countries don't have voluntary military alliances. The US had (past tense) the largest and most effective set of voluntary military alliances in the world, and that was the heart of American power that Donald Trump is dismantling.

The why is an interesting question. I personally don't believe it's all ideology and idiocy. There has to be a level of compromise.