A year after an explosion ripped apart the 1,200-ton Cheonan warship, killing 46 sailors and horrifying Seoul and its allies, the sinking still enrages and divides South Korea. It also may have taught North Korea a dangerous lesson. Demonstrators in Seoul this week demanded revenge against the North, and the South Korean government and its political opposition traded barbs on the sinking. Seoul has suggested skeptics of the North’s role are weakening national security. South Korean citizens and lawmakers still question a Seoul-led multinational investigation that concluded a North Korean submarine torpedoed the warship. Pyongyang rejected the finding, and China shielded its ally at the United Nations. Eight months after the Cheonan sank, a defiant North Korea killed two civilians and two marines in an artillery attack on South Korea’s Yeonpyeong Island in disputed waters, fueling fears that the North now thinks it can get away with crossing lines once thought uncrossable. “They’ve pushed the red line back,” Joseph Bermudez, a North Korea military analyst for the London-based Jane’s Information Group, said of the North. “Let’s face it, you sunk somebody’s warship, then you shelled their island, and have (South Koreans) retaliated in any way that’s significant? Not really.” In the past year, the North has given its apparent leader-in-waiting a slew of high-power jobs, proudly unveiled a new nuclear facility that could give it another way to make atomic bombs and rained artillery shells on South Korean civilians. But the Cheonan sinking still resonates. Hundreds of protesters in Seoul carried signs Friday that read “Revenge” and “Never forget Cheonan”; they torched pictures of North Korean leader Kim Jong Il. President Lee Myung-bak on Saturday burned incense and offered flowers at a memorial ceremony attended by more than 4,500. Family and friends wept in front of pictures of those killed. The North’s state media fanned the flames Saturday, calling the warship sinking a “self-fabricated scenario” by South Korean conservatives, with the United States pulling “the strings from behind the scene.” Lee has lashed out at domestic doubters, expressing sadness on the eve of the sinking anniversary that “among the people who distorted facts just as North Korea did at the time, there has been nobody who had the courage to confess their mistakes,” according to the president’s office. Earlier this month, Lee’s government released a video that blamed public disputes over the Cheonan sinking for the Yeonpyeong shelling and warned that continued discord could cause even more attacks. Liberals were furious. The left-leaning Hankyoreh newspaper said in an editorial that it’s “groundless to attribute the shelling on Yeonpyeong Island to the debate over the sinking of Cheonan warship.” A senior opposition lawmaker, Park Jie-won, said this week that South Koreans still have doubts about the cause of the sinking and called on the government to “resolve suspicions.” Public opinion, however, seems solidly in agreement with the government amid widespread anger and shock in the South over the civilian deaths on Yeonpyeong. A government-commissioned survey released this week by private Hankook Research showed eight in 10 South Koreans believe North Korea was responsible for the sinking. The telephone survey of 1,000 South Korean adults had a margin of error of plus or minus 3.1 percentage points. As the Koreas — still technically in a state of war — square off, the Cheonan will continue to be a sore point. The South demands a “sincere apology” for last year’s attacks. Without Pyongyang taking responsibility, South Korea won’t consider serious talks. And until Seoul is satisfied, the United States won’t push for a resumption of stalled international negotiations meant to end North Korea’s nuclear weapons programs. No diplomatic progress makes the possibility of a third nuclear test high. Seoul, which has scrambled to plug military holes and boost troops and weapons, isn’t letting North Korea forget Saturday’s anniversary: It staged live-fire drills Thursday near the border, massing more troops and weapons than normal because of the anniversary. The navy also planned firing exercises Saturday. Pyongyang warned this week that “a war may break out anytime.” Domestic politics in each country will complicate ties. Deep divisions over North Korea policy will be highlighted as South Korea enters its presidential election season next year. In North Korea, the government is working to pave the way for Kim Jong Il’s apparent power hand-off to his youngest son, Kim Jong Un. It’s also preparing for next year’s centennial of founder Kim Il Sung’s birth, when the North has vowed to become a powerful and prosperous country. A year after the Cheonan sinking, friction between North and South Korea is constant. “It’s very easy to catch a spark, so it would not be a surprise to see another conflagration,” said John Delury, an assistant professor at Yonsei University’s Graduate School of International Studies.