CONCERNS about terrorism have begun to have an impact on many aspects of American life and have prompted a visible increase in security in this city. But the recent terrorist attack in London is a reminder that for the British, potential dangers in connection with the Gulf war are an additional and not a new burden. The Irish Republican Army, which has claimed responsibility for the shelling in Downing Street, has been engaged in a campaign of terror for more than 20 years. This warfare has cost thousands of lives in Ulster and has killed civilians as well as government officials and military personnel in England. It is extremely fortunate that this assault, in which shells landed just 40 feet from the room in which the prime minister and his war cabinet were meeting, did not kill anyone.

This latest act of terror is also another indication that the IRA has its own agenda, which includes the undermining of any step toward reconciliation that might bring peace to Ulster. On the very day of the attack, for example, a British appeals court took the first step toward reopening the notorious case of the Birmingham Six. Defendants in that case, Irishmen convicted of a bombing in which dozens were killed, have sought reconsideration for years, claiming that the forensic evidence against them had been forged and their confessions coerced. New evidence supporting their claims was unearthed by the Cornwall and Devon police, and the British home secretary just recently reopened the case.

Some experts say that though the IRA attack failed to maim or kill, it will be counted a success if it undermines morale and frightens the civilian population. But after the shells shattered his windows and cratered his garden, Prime Minister John Major calmly resumed his meeting in another room and continued to get on with the nation's business. That kind of resolve and fortitude is the best response to all the world's terrorists.