By Jeff Jacoby

 ONE OF THE FIRST people I met during a week's stay in Havana last year was the economist
 Oscar Espinosa Chepe, a once-ardent communist who had turned against Fidel Castro's dictatorial
 system. For daring to criticize Cuba's disastrous policies, Chepe and his wife Miriam had been
 severely punished. He lost his prestigious position in the foreign service; she was told to choose
 between her job in the Foreign Ministry and her marriage. ''You want me to divorce my husband?''
 she had asked in disbelief. ''Well, it's up to you,'' came the reply. As we sat in their tiny apartment in
 Havana's Playa district last March, the two dissidents told me how they had been forced to sell
 many of their possessions -- the car, the stamp collection, even some of their clothes -- in order to
 keep body and soul together. Barred from normal employment, Chepe managed to cobble together
 an income from odd jobs teaching and writing about Cuba's dysfunctional economy.

 Now he will be unable to do even that. Chepe was one of nearly 80 Cuban dissidents seized in
 mass arrests across the island last week. After a summary trial on Monday he was convicted on
 trumped-up charges of ''working with a foreign power to undermine the government.'' His
 punishment was 20 years in prison.

 Also arrested, tried, and convicted this week was Marta Beatriz Roque, another intellectual who
 went from believing in Castro's communist revolution to acknowledging its utter failure. Her calls for
 reform got her fired from the University of Havana faculty and, like Chepe, she decided to work as
 an independent economist, disseminating through unofficial channels the grim facts about life in
 Cuba.

 On the afternoon that I visited her meager flat, Roque welcomed me cheerfully, glad of the chance
 to practice her English. She showed me the gouges on the door frame where the police had recently
 broken into her house. ''They took everything I could use to write,'' she laughed. ''Even my pencil --
 and every piece of paper.''

 She assumed she was under surveillance, she said, handing me an espresso her secretary brought
 from the kitchen, but she wouldn't stop now. She had already spent more than two years locked up
 for spreading ''enemy propaganda'' -- the regime's term for accurate statistics -- and wasn't going
 to worry about what the future might bring.

 On Monday, it brought a 20-year sentence. The chief witness at her kangaroo trial was the
 government agent who had spied on her every move: her secretary.

 I met Hector Palacios when I went see the tiny lending library maintained by his wife in their
 cramped third-floor walk-up. (In Cuba, lending books is also a crime.) Ninety percent of Cubans
 no longer believe anything Castro says, Palacios estimated, and if they were free to leave, 5 million
 of them would do so. Formerly an official in the Communist Party, he had soured on the
 government in 1980 when he saw people beaten in the streets for wanting to emigrate.

 If he could send a message to the American people, Palacios was asked, what would it be? ''I
 would tell them that there are two embargoes affecting Cuba,'' he said. ''There is the US economic
 embargo against Cuba. And there is Castro's embargo against the Cuban people.''

 For engaging in peaceful dissent, Palacios was sent to prison twice in the 1990s, each time for 1
 1/2 years. The latest wave of repression has just swept him behind bars again -- this time for 25
 years.

 Champions of ''constructive engagement'' have long insisted that the surest way to bring freedom
 and democracy to Cuba was to flood the island with tourists and foreign trade. They have loudly
 blasted the US embargo, which restricts Americans' freedom to travel to Cuba or do business
 there. Their minds have not been changed by the fact that hundreds of thousands of tourists and
 hundreds of millions of dollars already surge into Cuba annually, all without appreciably increasing
 the liberty of ordinary Cubans. Most of the influx is Canadian and European, but a significant chunk
 is American: 80,000 US citizens travel to Cuba each year via a third country.

 Every few years Castro unleashes a brutal crackdown, sweeping scores of innocent victims --
 dissidents and democrats guilty of nothing more than thinking for themselves -- into his dungeons. It
 isn't something he does because he has been insufficiently exposed to commerce and tourism, or
 because he resents the US embargo, or because Jimmy Carter and other credulous liberals haven't
 lavished him with his usual quota of flattery.

 He does it because he is a ruthless tyrant who craves power more than anything else. For 44 years
 he has let nothing weaken his stranglehold on Cuba, and neither concessions nor sanctions nor
 international condemnation will change his behavior now. The only one way to reform a totalitarian
 despot like Castro is to topple his regime. Peacefully if possible, by force if necessary.

 Jeff Jacoby's e-mail address is jacoby@globe.com.