The Pentagon believes that ISIS is weakened. They point to its rather easy loss of Manbij and Jarablus.
They are dreaming. ISIS didn't lose Jarablus. The Turkish army occupied it for them so that the Kurds can't touch it. The ties between Turkey and ISIS are old and there is no reason to believe that Erdogan's talk about fighting ISIS as an excuse for his invasion in Syria is anything else than lies. The "conquest" of Jarablus happened almost without a fight and recent reports show a lot of fighting against Kurds, but none against ISIS. So there is no reason at all that the "fall" of Jarablus weakened ISIS.
Also, if ISIS had experienced the Turkish invasion as a major threat one would have expected them to send troops to the north of Syria to fight the Turks. Yet it looks like the opposite is happening. ISIS is attacking the Kurds south of Manbij and it has also started a major offensive against the Syrian government in the Palmyra region.
The story of Manbij is a more complex one. Once upon a time ISIS fought tooth and nail to occupy Kobani. They were on a winning streak and they didn't want to lose their aura of invincibility. In the end they had to conclude that this was foolish: they sent scores of men into death without any chance of success. Since then they have given up cities rather easy when coming under pressure. In many cases - like Manbij, Ramadi and Fallujah - they cut an agreement with their opponents that gave them the opportunity to withdraw. I don't know about Palmyra, but I wouldn't be astonished if there had been some kind of agreement there too when ISIS lost control.
ISIS is following standard a insurgent strategy, as described by Mao and others. They occupy cities when they see a chance and they appreciate the propaganda benefits of such actions. But their roots are in the countryside from where they can attack when it suits them. They know better now than holding cities at all costs.