Oleg Sentsov stands behind bars as his verdict is read at a court in Rostov-on-Don, Russia, in August 2015. (Uncredited/AP)

FOUR YEARS after Russia invaded and illegally annexed the Crimea region of Ukraine, the crime and its victims get little attention. One of them, a courageous activist and filmmaker named Oleg Sentsov, is determined to change that. On May 14, Mr. Sentsov, who is imprisoned in a Siberian penal colony above the Arctic Circle, embarked on a hunger strike to demand the release of 64 Ukrainian political prisoners held by Russia; he did not include himself. Since then, his health has seriously deteriorated. His lawyer says he has lost about 17 pounds and his vital organs are in danger of failing. If the strike continues, he could die during the upcoming World Cup soccer championship, which Russia is hosting.

The regime of Vladi­mir Putin ought to be shamed into freeing Mr. Sentsov before such a tragedy occurs. As human rights groups have meticulously documented, the 41-year-old native of Simferopol, the Crimean capital, is imprisoned because he resisted the forcible Russian occupation of his country. After he coordinated relief efforts for Ukrainian soldiers trapped on a Crimean base, Mr. Senstov and an associate were arrested in May 2014 and charged with terrorism.

Authorities claimed Mr. Sentsov set fire to the door of the office of a Russian political party. Mr. Sentsov denied the charge and said he had been tortured in prison. Officials answered by cynically describing his bruises as the result of his interest in sadomasochistic sex. At his trial, the prosecution’s main witness recanted, saying his testimony had also been extracted under torture. Yet in August 2015, the court sentenced Mr. Sentsov to 20 years in prison in the Yamalo-Nenets Autonomous District, about 2,000 miles from Crimea.

Russia followed the invasion of Crimea with another of eastern Ukraine, which for the past several years has festered in stalemate. Mr. Putin has been offered plenty of chances to settle, by removing Russian troops from the eastern Ukrainian provinces of Donetsk and Luhansk; he chooses not to. Instead, he continues to maliciously meddle in Ukraine and in Western democracies, through cyberoperations and acts of violence, such as the recent attempted assassination of a former spy and his daughter in Britain.

The countries that will dispatch teams and fans to Russia for this summer’s World Cup will bestow prestige on Mr. Putin in spite of this history — and in spite of his regime’s record of doping its own athletes. Their political leaders ought at least to insist that Mr. Putin release Mr. Sentsov and other Ukrainian prisoners. To their credit, some are doing so: French President Emmanuel Macron raised Mr. Sentsov’s case both in private and publicly when he recently met with Mr. Putin.

Ukraine, for its part, has floated the idea of a prisoner exchange that would free 23 Russians convicted of crimes in Ukraine for Mr. Sentsov and several other prisoners. Mr. Putin would be wise to take the deal. Otherwise, he may find himself being asked to explain the death of a man who never should have been brought to his country, much less imprisoned there.