"In a sign of volatile East-West relations, a dispute over shopping in the Ukrainian conflict zone spiraled into fighting that sent war jitters rippling through Europe and the U.S.Across the border, Russia scrambled jets. The next day, Russian tanks mounted on rail cars rumbled toward the Ukrainian border. Diplomacy in Berlin, Moscow and Washington went into high gear. The sudden spike in hostilities last month underscored the tenuous nature of the cease-fire that exists along the 279-mile front in the Ukraine war. It set off a new round of ominous warnings from Moscow, and highlighted President Vladimir V. Putin’s willingness to escalate what is known as hybrid conflict, a blend of military and other means for creating disruption — including exploiting humanitarian crises like the current one on the Polish-Belarusian border."