The tolerance for troop losses tends to decrease as urban warfare drags on. In Mosul, after casualty rates for Iraqi troops reached 50 percent, airstrikes increased. The Iraqis often demanded that their American partners take down more than just one building.
“They wouldn’t move forward until we leveled the block,” said Amos Fox, a U.S. military planner in Iraq during the Mosul operation.
It took 252 days and 100,000 Iraqi troops, with American air support, to rid Mosul of ISIS. Along with 10,000 civilians, 8,200 Iraqi soldiers died, and at least 13,000 buildings were made uninhabitable, according to the United Nations.
Israeli troops, while better trained than the Iraqi Army, with years to prepare invasion plans and special engineering units for challenges like tunnels, will face a tougher adversary.
There were 3,000 to 5,000 ISIS fighters in Mosul at the start of the operation, according to initial U.S. military estimates, and a few thousand more came later. Hamas’ military wing, the Al Qassam Brigades, is estimated to number 30,000 to 40,000, not including thousands of militants with other groups like Palestinian Islamic Jihad.
Most ISIS fighters were also from other countries. Hamas fighters grew up in Gaza. Their units are bonded together by location, family, faith and shared frustration with Israel.
“They know the streets, they know the tunnels,” said Mr. Blish, the retired lieutenant colonel. “It’s going to be a tough slog.”
Recent wars have rarely gone as quickly as either side expected: Mosul was supposed to be cleared in three months, not nine; Russia expected a rush to victory in Kyiv. And as the death toll mounts and economies crater, international support often weakens.
Ukraine’s leaders know this better than most, but Israel also has experience with the political and military challenge that comes with extended conflict. Israel’s last ground invasion in Gaza, in 2014, lasted less than three weeks. As Mr. Spencer at West Point’s Modern War Institute recently pointed out, Israel “has fought almost every war of its history in a race against time, seeking to achieve its goals before international pressure forces it to stop operations.”
In the current case, Israeli officials have warned that they are expecting several months of fighting, if not years. That would mean a spike in demand for weapons, and added global volatility. American officials are worried about the potential for the war to spread to Lebanon and Iran, along with possible attacks on American troops in Iraq. Those concerns could affect how Washington advises Israel as the conflict evolves.