The context was just before the battle of Jericho, when the children of Israel were about to finally begin to receive a promise made to them hundreds of years before, after having wandered in the desert for forty years. Given the circumstances, one would think that the commander of the Lord's army would tell Joshua that he was on Joshua's side. Yet, his answer was that he wasn't on anyone's side but the Lord's.
The lesson here is that God's purposes are transcendent: God does not adopt our causes as his own, and when he does battle it is not on our behalf, but on his own behalf. Since God does battle for himself and his will alone, it follows that he is not offering us a blank check or is offering to battle for us even when we are wrong. Rather, to find success we need to discover his will and align with it, adopting his spiritual battle as our own.
In the current political situation in the US, this means that God is not on Trump's side, or on Biden's side, for that matter: God is on his own side, defending his will, his promises, and his righteousness. While this does nothing to assure us as to what will happen in the short of even the mid-term, it should bring comfort to us to know that in the end, God's will will prevail.