Russia is also building miles of trenches, and pillboxes — small structures for their troops to shoot from.

The fortifications could slow Ukraine’s army — but they are effective only if manned correctly.

If the positions are unmanned, they are useful only if there is an orderly Russian retreat, one of the most difficult tactical operations to conduct, said Philip Wasielewski, a fellow at the Foreign Policy Research Institute.

“There’s no guarantee that those soldiers will make it to those fortifications. Or once they get there, not just keep running,” Mr. Wasielewski said. “These are just holes in the ground unless they are held by motivated, disciplined soldiers, who are supported by artillery, mobile reserves and logistics.”

[The collapse in Kherson Oblast was facilitated by Russian underestimation of Ukrainian capabilities in the open field. They appear to have learned from that humiliation, but the morale of their forces will not sustain a lengthy trench-warfare strategy either. Especially not in the middle of a Ukrainian winter. — Ed]