BERLIN—For three years, a crack team of detectives gathered each weekday morning around a whiteboard at the German Federal Police headquarters in Potsdam, near Berlin. Now their investigation into who was behind the greatest act of sabotage in modern history—the bombing of the Nord Stream pipelines—is threatening to splinter support for Ukraine, the country they hold responsible.Poland already has refused to extradite one of the suspects to stand trial in Germany. It instead views him a hero for destroying a vital source of revenue for Russian President Vladimir Putin’s war machine. Polish Prime Minister Donald Tusk, who has long questioned...