WHOEVER failed to erase the words “Number One” in blue Korean script etched inside the propulsion shaft of a deadly torpedo may well be in deep trouble in Pyongyang. On May 15th a ship dredging the site of the attack on a South Korean warship in March that killed 46 seamen made a spectacular find: propellers, motors and a steering section that international investigators say “perfectly match” those of a CHT-02D torpedo that North Korea sells abroad. What's more, the blue marking was similar to one on a previously captured North Korean torpedo. This was as close to a smoking gun as the South Koreans could have hoped to find.

The discovery, combined with intelligence reports indicating North Korean submarines were out of port during the attack, allowed the investigators to conclude on May 20th that the Cheonan “was sunk as the result of an external underwater explosion caused by a torpedo made in North Korea.” Or as one person close to the investigation succinctly put it: “It was either the North Koreans, or it was the Martians.”

The South Korean government of President Lee Myung-bak had barely dared hope to find hard evidence of North Korean guilt, however strong its suspicions. In doing so, it pushes the belligerent regime in Pyongyang into a corner. True to form, it continued to deny its involvement on May 20th, calling the investigation a “fabrication” and threatened “all-out war” if new sanctions were imposed.

Diplomats say it will also make it harder for North Korea 's main ally, China, not to accept the regime's culpability. A senior official in Beijing reportedly called the incident “very unfortunate”. He did not comment on the investigation, though an official in Seoul said China's ambassador was briefed on the results on May 18th.

In coming days China will come under pressure to issue a tougher response, during what could be some uncomfortable diplomatic meetings. Hillary Clinton, America's secretary of state, is to visit Beijing on May 24th and 25th. Following that, the leaders of China, South Korea and Japan are scheduled to hold a summit in South Korea on May 29th and 30th.

The proof of North Korean involvement, say diplomats, may help South Korea and its allies persuade China to back some form of United Nations condemnation of the attack. South Korea's allies such as America, Japan, Britain and Australia were quick to express their outrage. But the toughest condemnation, in the form of a Security-Council resolution, needs China's support, or at least abstention.

Beyond diplomatic measures, South Korea's retaliatory options are relatively puny—and dangerous. South Korean diplomats suggest that the government may announce a halt to remaining business ties with the North, except through the cross-border industrial complex, known as Kaesong. It may close a shipping shortcut to North Korean vessels, and conduct joint training exercises with American maritime forces in the Yellow Sea.

A retaliatory strike, which some hawkish South Koreans are calling for, is something the pro-business President Lee is considered reluctant to do for fear of escalating the conflict. His party faces local elections that it is expected to win on June 2nd (his liberal opponents already accuse him, perhaps unfairly, of using the incident for political ends). After that he hosts an important G20 summit in November.

Among the population at large, there seems little stomach for a fight. Indeed, despite a national outpouring of grief, the senseless attack aroused surprisingly few public demonstrations of wrath with the North. Brian Myers, a writer on North Korea, notes that there was more palpable anger in 2002 when an American army vehicle ran over two South Korean schoolgirls.

However, he and other North Korea watchers think tough measures may eventually be called for. The more the cash-strapped regime in Pyongyang feels economically isolated, the more it is likely to step up its provocations against the South unless it is decisively put in its place, Mr Myers says. As it is, the North is in desperate economic straits. Six-party talks that aimed to encourage it to give up nuclear weapons in exchange for cash are stalled indefinitely as a result of the torpedo attack. International sanctions have hit other sources of hard currency.

China is its economic lifeline, as was shown again when Kim Jong Il, North Korea's dictator, made a hasty trip to Beijing not long after the Cheonan incident to beg for continued economic support. At that point, Beijing had an excuse, in public at least, to take his claims of innocence at face value. It no longer does.