After weeks of being completely upbeat about the invasion of the ISIS capital city, Pentagon officials today confirmed that the invasion of Raqqa by US-backed Kurdish forces, and some US forces themselves, is slow going, facing a lot of resistance and stop-and-go advances.

To make matters worse, Pentagon spokesman Capt. Jeff Davis confirmed “heavy casualties” being sustained on the Kurdish side, though he did not offer any specifics on exactly how many casualties that would mean for a force of 30,000 to 40,000 fighters.

This must immediately draw comparisons to the US-backed invasion of Mosul, which officials were similarly upbeat about, and which also stalled into an increasingly ugly brawl, with not only heavy losses among combatants but a disastrously high civilian toll.

The US assumption seems to be that they can ultimately wait out ISIS forces, and while that may ultimately be the case, the costs of doing so, as shown in Mosul, can be dreadfully high, and with the Kurdish YPG already facing growing fighting against Turkey further to the north, it’s not clear they’ll be able to commit to a long-term, casualty-heavy fight against ISIS over Raqqa, a city the US has promised the Kurds won’t get to keep even if they do capture it.