It has been a summer of good news for Ukraine.

Its drone strikes deep inside Russia have created gasoline shortages, strained Russia’s economy, stoked anxiety among ordinary Russians, and created headaches for Vladimir Putin. Ukrainian forces have effectively isolated occupied Crimea from the Russian mainland, and they continue to strike Russian vessels in the Black Sea and the Sea of Azov.

But Russia has scored some points of its own recently, and not just by terrorizing Ukrainian civilians with long-range missiles. A newly intense campaign against Ukraine’s Black Sea ports is exposing a shortage of air-defense interceptors, disrupting grain exports, undermining export revenue, and halting arms deliveries to Ukrainian forces. Agricultural products account for 60% of the country’s goods exports, and Russia’s attacks come just as Ukraine’s farmers prepare for the harvest season.

If the disruption lasts, Russia could choke off a crucial source of foreign currency for Ukraine and boost Russia’s........

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