MYKOLAIV REGION, Ukraine — A platoon of U.S. Marine reservists with the 2nd Battalion, 25th Marine Regiment trained with Ukrainian marines to attack a fictitious foe, roughly 100 miles from the Russian-occupied Crimean Peninsula. The purpose of Thursday’s exercise was to increase combat effectiveness and demonstrate resolve among the 17 allied and partner nations participating in the training known as Exercise Sea Breeze, now in its 18th iteration. Russia’s annexation of Crimea in 2014 and the subsequent conflict in eastern Ukraine have led to a sharp increase in tensions throughout the region. Moscow warned on Thursday that the multinational maritime...