I just realised whats happened, and what is the target of the USA .
They disabled the bridges over the Dniper, the target of this wasn't to restrict the arrival of reinfocement, but the oposite, the slow down the withdrawal of them.
As soon as the reinfocement arrived to the Southern offensive, they started the attack in the North.
Now, there is no free reserve to quickly cover the hole, so they have a good playground in the north to do whatever they wish.
Ultimatedly, the Russians will get back the area, but the USA could achive one or several of the next objectives:
1. Increase the losses of the Russians
2. Distrupt the rebuilding of the russian controled areas
3. increase the cost of the war for the Russian economy.