Slowly but steadily this summer, Russian troops are forging through Ukraine’s outgunned and undermanned defenses along the front line of the battlefield in a relentless onslaught, prompting the West to push for new weapons and strategies to shore up Kyiv. That, in turn, has brought new political demands by President Vladimir Putin of Ukraine’s leadership and threats of retaliation against the West — either directly or by indirect methods. The moves by the West to blunt the offensive and the potential Kremlin response could lead to a dangerous escalation as the war drags through its third year — one that further raises the peril of a direct confrontation between Russia and NATO.