MOSCOW—Police detained prominent Kremlin critic Alexei Navalny on arrival at a Moscow airport on Sunday after he flew home to Russia from Germany for the first time since he was poisoned last summer. The move, which could see Navalny jailed for 3.5 years for allegedly flouting the terms of a suspended prison sentence, is likely to spark a wave of Western criticism of President Vladimir Putin. In a case that drew wide international attention, Navalny was poisoned last summer by what German military tests showed was a Novichok nerve agent, a version of events the Kremlin rejects. Navalny’s plane from Berlin was diverted to another Moscow airport at the last minute in an apparent effort by authorities to thwart journalists and supporters greeting him. After Navalny said last week he planned to return home, the Moscow prison service (FSIN) said it would do everything to arrest him once he returned, …