Trump is now demanding that NATO help the United States reopen the Strait of Hormuz, a vital passage through which 20 percent of the world’s oil travels, which Iran has effectively closed in response to the United States’ and Israel’s attacks. NATO has refused. And though Russia is playing only a tangential role in the crisis there, Moscow could emerge as one of the victors of war. The standoff over the strait has created an unexpected new strain on the relationship between Trump and NATO, one that may not fully shatter the alliance but could fundamentally weaken it.