Senior Ukrainian officials stepped up intelligence sharing with their American counterparts over the summer as they began to plan the counteroffensive that allowed them to make dramatic gains in the northeast in recent days, a shift that allowed the United States to provide better and more relevant information about Russian weaknesses, according to American officials. Throughout the war, the United States has provided Ukraine with information on command posts, ammunition depots and other key nodes in the Russian military lines. Such real-time intelligence has allowed the Ukrainians — who U.S. officials acknowledge have played the decisive role in planning and...