Terrorists brought down a plane over Lockerbie, Scotland, in December 1988

From Correspondent Jim Clancy

(CNN) -- The bomb blasts at U.S. Embassy compounds in Nairobi, Kenya, and Dar es Salaam, Tanzania, demonstrate the continued vulnerability of U.S. interests around the world, despite years of anti-terrorist efforts and millions of dollars spent to improve security.

A Kenyan doctor summed it up: No one expected an attack in his country, an attitude which may have made the U.S. Embassy there all the more vulnerable to a terrorist with an agenda.

The last major bombing of a U.S. target abroad was in June 1996, when a fuel truck loaded with explosives shattered a U.S. military complex in Khobar, Saudi Arabia. Nineteen people were killed and more than 400 others were wounded.

Responsibility for that bombing has never been resolved to the satisfaction of U.S. investigators.

When it comes to attacks on U.S. interests abroad, airlines have been one of the more frequent targets.

In December 1988, a Pan Am jetliner on its way from Britain to the United States was brought down by a bomb over Lockerbie, Scotland, killing 259 people in the plane and on the ground. After one of the most exhaustive investigations in history, U.S. and British authorities laid the blame on Libya.

In April 1986, a TWA plane bound for Athens was hit by a small in-flight explosion that caused four passengers to be sucked out of the aircraft to their deaths. A year earlier, another TWA plane was hijacked and taken to Beirut. One U.S. serviceman was killed, but the other passengers were released after a two-week drama.

A few months later, in December 1985, suicide squads simultaneously hit the Rome and Vienna airports, targeting Israeli and U.S. airline check-in counters.

Palestinian fighters shot it out with armed Israeli security agents in the crowded airports. Sixteen people were killed in the cross-fire, along with four of the terrorists.

But perhaps the deadliest terrorist attack ever carried out against U.S. targets abroad came in 1983, when suicide bombers penetrated U.S. and French military compounds in Beirut. The attacks killed 241 Americans and 58 French paratroopers.

The history of terror attacks demonstrates at least two things. First, terrorists, if well-organized and willing to die for their cause, can overcome most security precautions.

And second, if terrorists are less organized and unwilling to give up their own lives, they can find a softer target in a country where terrorist attacks aren't anticipated -- such as Kenya and Tanzania.