This week’s first exports of Ukrainian grain under a deal brokered by the United Nations and Turkey are a joyous headline for Ukraine and the many countries where the global surge in grain prices has caused food shortages. The deal could reduce prices if shipments accelerate, but it is vulnerable, as Russia signaled days ago by slamming missiles into Ukraine’s biggest seaport hours after formally agreeing to let that port ship its grain stocks. This fragile deal offers cautions for policymakers pondering eventual negotiations with Moscow to help end the Ukraine war and rebuild security in Europe.