I doubt if most people these day remember the Domino Theory. It was the main reason the U.S. got involved in Vietnam.
The domino theory, which governed much of U.S. foreign policy beginning in the early 1950s, held that a communist victory in one nation would quickly lead to a chain reaction of communist takeovers in neighboring states. In Southeast Asia, the United States government used the domino theory to justify its support of a non-communist regime in South Vietnam against the communist government of North Vietnam, and ultimately its increasing involvement in the long-running Vietnam War (1954-75). In fact, the American failure to prevent a communist victory in Vietnam had much less of a global impact than had been assumed by the domino theory. Though communist regimes did arise in Laos and Cambodia after 1975, communism failed to spread throughout the rest of Southeast Asia.
If we replace "communism" with "Islamism" we can dust off the Domino theory and put it into use again.
Think Afghanistan instead of Vietnam...mmm
Think Mali and Algeria instead of Vietnam...
Since there's lots of oil involved in the Middle East, there's an even greater incentive to consider self interests and revive the Domino theory.
If Islamists governing Egypt now wasn't a wake up call, then consider the unrest in other nations such as Mali and Algeria.