Scanning a map of the world must give President Obama a sinking feeling as he contemplates the dismal state of troubled bilateral relationships his administration has sought to turn around. He would be smart to take a hard look at Cuba, where a major policy shift could yield a significant foreign policy success.

For the first time in more than 50 years, shifting politics in the United States and changing policies in Cuba make it politically feasible to re-establish formal diplomatic relations and dismantle the senseless embargo. The Castro regime has long blamed the embargo for its shortcomings, and has kept ordinary Cubans largely cut off from the world. Mr. Obama should seize this opportunity to end a long era of enmity and help a population that has suffered enormously since Washington ended diplomatic relations in 1961, two years after Fidel Castro assumed power.

In recent years, a devastated economy has forced Cuba to make reforms — a process that has gained urgency with the economic crisis in Venezuela, which gives Cuba heavily subsidized oil. Officials in Havana, fearing that Venezuela could cut its aid, have taken significant steps to liberalize and diversify the island’s tightly controlled economy.

They have begun allowing citizens to take private-sector jobs and own property. This spring, Cuba’s National Assembly passed a law to encourage foreign investment in the country. With Brazilian capital, Cuba is building a seaport, a major project that will be economically viable only if American sanctions are lifted. And in April, Cuban diplomats began negotiating a cooperation agreement with the European Union. They have shown up at the initial meetings prepared, eager and mindful that the Europeans will insist on greater reforms and freedoms.