I don't know.

I think that the Democrats will almost certainly use the three largest tools in their arsenal right away. There will be a War Powers resolution, there will be legislation limiting the use of funds for the war, and they'll start impeachment proceedings against Hegseth. None is likely to have much effect on the prosecution of the war, if Trump doesn't want them to. He'll ignore the WP reso (claiming it unconstitutional), veto the spending provision, and Hegseth will just have to deal with an uncomfortable time in the barrel. Although the increasing chatter about Hegseth failing up to a Senate seat in TN suggests that Trump might be at least considering a major pivot on the war.

Congress does have one actual weapon against the war - they can redo those spending limits in a must-pass bill, which Trump might be less able to block with his veto. But that's some time down the road.

As for policy more generally, the Blue Wave ended up neutering most of the Congressional caucuses. Speaker Jeffries will have so large a working margin (more than 30 votes to lose) that none of the individual blocs within the party will have anywhere near as much influence as they might have hoped. So look for House policy to hew far closer to where the Speaker wants it to go. Progressives will have a lot more power if/when the Democrats lose a bunch of their new seats - the seats that get lost when a wave recedes will be in the purpler districts, which is going to be more of the moderates and fewer of the progressives. But that's not happening until 2028 at least, and even then perhaps not if Trump remains singularly unpopular.