Over the past several years, the United States has embarked on a program of secret research on biological weapons that, some officials say, tests the limits of the global treaty banning such weapons. The 1972 treaty forbids nations from developing or acquiring weapons that spread disease, but it allows work on vaccines and other protective measures. Government officials said the secret research, which mimicked the major steps a state or terrorist would take to create a biological arsenal, was aimed at better understanding the threat. The projects, which have not been previously disclosed, were begun under President Clinton and have been embraced by the Bush administration. During the cold war, both the United States and the Soviet Union produced vast quantities of germ weapons, enough to kill everyone on earth. Eager to halt the spread of what many called the poor man's atom bomb, the United States unilaterally gave up germ arms and helped lead the global campaign to abolish them. By 1975, most of the world's nations had signed the convention. In doing so, they agreed not to develop, produce, acquire or stockpile quantities or types of germs that had no prophylactic, protective or other peaceful purposes." They also pledged not to develop or obtain weapons or other equipment "designed to use such agents or toxins for hostile purposes or in armed conflict." The pact did not define "defensive" research or say what studies might be prohibited, if any. And it provided no means of catching cheaters.