It's a good thing Brian Jenkins is on our side. For nearly three decades, first on countless index cards and now in 15,000 computer files, he has amassed an arsenal of information about the violence known as terrorism, intended to create fear. Jenkins knows more about terrorism than most terrorists. He is a senior adviser to the president of the RAND Corporation in Santa Monica, a nonprofit, nonpartisan research institution that provides information and analysis to the government and to corporations. Don't let his easy laugh fool you, or his soft-spoken manner, or his artistry with oils on canvas. Jenkins, who is 68 but looks 55, is a former army paratrooper and Green Beret captain whose every knuckle has been broken and whose jaw whiskers have all the warm fuzziness of steel wool. A Fulbright fellow, he also is a decorated combat veteran who served in the Dominican Republic and three-and-a-half years in Vietnam.