ANOTHER BIG set of riots has hit South Korea. It's the opposition's way to show the world what it thinks of the government run by President Chun Doo Hwan. The street opposition's tactics are rough stuff -- taunting and provoking the (unarmed) police. But the police use rougher stuff: sometimes prisoners leave detention dead. The intensity of the protests and of the official response to them appears to be mounting.

There is no mystery to what is going on. The military runs South Korea, through generals who have taken off their stars, offering economic progress and anticommunism to legitimize their rule. Increasingly, however, middle-class Koreans demand a more democratic arrangement. In the current cycle of unrest, the issue has been whether to move from the old electoral system favoring the military's comforts and candidates, to one giving the political opposition a wider opening. President Chun, admittedly sorely tried, this spring made the fundamental error of breaking off talks on constitutional revision, closing off the principal safety valve. It was his subsequent anointment of another former general, Roh Tae Woo, under the old system, that occasioned the protests that turned nasty the other day.

Americans ask why a country that received decades of protection and patronage is not more democratic. The Seoul regime has some curious responses. The Korean Embassy's cultural and press attache, for instance, meaning to rebut this newspaper's criticism of Korea's military style in politics, recalled in a letter to the editor in April that the two last presidents had both resigned from the army before running for office. ''Let me add,'' he wrote, ''that I have never heard any American refer to President Eisenhower's two terms in office as a period of military rule in America.'' Nor have we. That a Korean would present President Chun and President Eisenhower as two of a democratic kind suggests a gulf in understanding not easily bridged.

Korea is a friendly and, in important respects, admirable country living in dangerous circumstances that compel special deference. But it is wrong and unnatural that the generals so cramp political freedom. Many political steps are available for President Chun to move toward democracy, stability and continued closeness to the United States. At the moment he is moving South Korea away from all three toward danger and chaos.