An explosion that sparked a fire on a bridge to Crimea caused massive damage to the train tracks and roadway, triggering a shakeup of security by Russia and jubilation among Ukrainian officials. The blast damaged a key supply route for Moscow’s floundering military campaign in Ukraine. It was also a major symbolic blow as the bridge connecting mainland Russia to the occupied Crimean peninsula was opened by President Vladimir Putin himself to much pomp and fanfare in 2018. The exact cause of the early Saturday blast remains unknown. Russia so far has said a truck exploded, without attributing r…