A Ukrainian air force MiG-29 fighter shot down one of the Russian air force’s best jets—an Su-35—near Russian-occupied Kherson on Friday, according to the Ukrainian defense ministry. The dramatic shoot-down, which reportedly took place as the Su-35 was chasing down Ukrainian attack planes, signaled an even more dramatic development on the ground. Ukrainian army formations supported by American-made M-777 howitzers crossed the Inhulets River around the town of Davydiv Brid, 50 miles northeast of Kherson. The Russians fell back to a trio of towns a few miles to the southwest. Towns whose defenses the Ukrainian general staff described as...