WARSAW—In a sprawling factory complex surrounded by derelict buildings, hundreds of technicians are working around the clock on one of the biggest challenges of Ukraine’s war: repairing artillery and heavy armor and returning it to the front line. Mechanics buzz around the football-field-sized workshop housing three AHS Krab guns, the air thick with the smell of metal dust and automotive grease. Two of the Krabs, which look like tanks but are self-propelled 155-caliber howitzer guns, are missing parts of their caterpillar tracks and are riddled with bullet holes and contorted metal.

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