The domino theory was a 20th Century foreign policy theory that speculated if one land in a region came under the influence of Communists, then more would follow in a domino effect. The domino effect indicates that some change, small in itself, will cause a similar change nearby, which then will cause another similar change, and so on in linear sequence, by analogy to a falling row of dominoes standing on end. (See butterfly effect.)Then at the end the article mentions
The theory was used by many United States leaders during the Cold War to justify U.S. intervention in the Vietnam War. The domino theory was applied by President Dwight D. Eisenhower and his top advisers in 1954 to describe the prospects of communist expansion in Asia if Indochina were to fall. Eisenhower argued that all of southeast Asia could fall. The theory's ultimate validity remained mixed, and debatable. After the U.S. left Vietnam, the North took over the South, and Cambodia and Laos had also turned to Communism, although Cambodia is no longer a communist state. This limited spread of Communism in Indochina provides ammunition for opponents of the theory, but both sides argue that the historical record overall supports their position.
In the 1980s, the domino theory was used again to justify the Reagan administration's interventions in Central America and the Caribbean region.
From its first conception, many have disputed central assumptions of the domino theory, for instance by arguing that Communist States lacked the tradition of cooperation the theory assumes (eg Cambodia attacked Vietnam, to which Vietnam responded by overthrowing the Khmer Rouge government). Supporters however have continued to argue it was a sensible policy in the context of the times.
Similarly, another new form of the domino theory has been advocated by those who seek to oppose Islamic terrorism. Some foreign policy advocates in the United States refer to the potential spread of both Islamic theocracy and liberal democracy in the Middle East as representing a sort of domino theory. During the Iran-Iraq war, the United States and many other western nations supported Iraq, fearing the spread of Iran's radical theocracy throughout the region. In the 2003 invasion of Iraq, neoconservatives argued that by invading Iraq a democratic government could be implemented, which would then help spread democracy and liberalism across the Middle East.
As I've argued 18 months ago, this push to democratize without first ensuring basic freedoms is not only useless, but counterproductive. In this case just the existence of democratic elections has not brought about the hoped for domino effect of democracy - it achieved gains instead for fanatical Islamic theocracy.
Radical Islam is the one place where the domino theory really does appear to exist. Thirty years ago, Islamism as a political movement was relegated to the fringes of the Arab and Muslim world, but since then (starting with the 1979 revolution in Iran) it has grown into a major worldwide political force, threatening not only the Western world but also the secular Arab world. It also has an advantage over Communism - the ascetism it demands and the religious virtues it claims makes it somewhat less vulnerable to internal dissent.
One cannot understate the fact that the goals of radical Islam is nothing short of world domination, with the infidels dead and the dhimmis subjugated.
This is why the war with Hezbollah is not just a regional conflict, not just a local spat. A victory (or perceived victory) by the terrorists is a huge boost tothe worldwide Islamicist movement in terms of recruitment and prestige. It is all too easy to imagine an Islamist revolution in Jordan, Egypt, Pakistan, Morocco and the Gulf states. And the idea that one would cascade into the next doesn't seem far-fatched - it seems inevitable.
So even if one tries hard to forget that Iran is racing to get the Bomb, the threat on the horizon is having a much-more empowered Iran or similar minded Islamists turn off the spigot of oil just to punish the West for not enforcing UN resolutions on Israel, or tolerating cartoons, or allowing R-rated movies, or any of a thousand other perceived injustices against Islam.