I was also very confused, so I looked it up now. So, here goes:

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So, to bend the knee means several things. In the context of this post it means submitting to authority, in this case specifically, doing whatever Trump tells to.

The flag has been a libertarian symbol for a few decades, but in the last five-ish years fascists and MAGAts have started using it so much that it gets associated with them by default.

Historically, that flag has been a symbol of the war for freedom from British rule.