Islam, of course, is a religion, a way of worshipping God with a set of rules seen as divinely ordained and a view of life flowing out of this theology and its actual practice over the centuries. As such, it is -- albeit with its own distinctive aspects -- much like other religions.

Certainly, it has had a close relationship with states over the centuries, but it has not been a ruling ideology, a blueprint for governing societies, for more than a dozen centuries, if even then.

In contrast, Islamism is a newly formulated revolutionary ideology for seizing state power and running all aspects of a country. It has developed virtually in living memory through the Muslim Brotherhoods and Iranian revolution through the Afghan war and down to al-Qaida. I can remember reading the works of Egyptian Islamists back in the 1970s when these were marginal figures heading small cults which were viewed by almost all Muslims as near-heretical. Today the Islamists often seem as if they are on the verge of gaining hegemony over Islamic thought, though even this falls short of gaining the support of most Muslims.

Today the idea of a global "class struggle" in which the Muslims (proletariat) is engaged in a battle against the evil Christians and Jews (capitalists, imperialists) to wage a revolution and create a new utopian Islamist state (communism) seems to be in the mainstream of the Arab and Muslim discussion. The radical Islamists have been able to shift the debate in the Arab world in their direction. Traditionalist clerics in many countries -- notably Egypt, Syria, and Saudi Arabia -- are saying things that a few years ago they would have condemned as being against Islam.

A lot of individual thinkers and activists contributed to this dizzying rise but I always come back to Ayatollah Ruhollah Khomeini as the central figure. The problem is that because he was a Shiite (not a Sunni) and an Iranian (not an Arab), Khomeini's direct influence must always remain limited. Yet remember that Khomeini was just one of a number of senior -- and not the most venerated --Shiite clerics in Iran. Many or most of these figures did not accept him as a leader or, as they say in Shiite Islam, a model whose ways should be imitated.

Khomeini's great accomplishment was not theological but political: to transform Islam into a revolutionary ideology. There were forerunners in Iran but much of Khomeini's achievement was to pose the Islamists as a revolutionary (Leninist) party. The ruling class group to be overthrown was dubbed as traitors to Islam. Their foreign allies or supporters were non-believing imperialists. The existing "anti-Islamic" system was equivalent to capitalism and the downtrodden Muslims were the proletariat. One could literally take the writings and actions of Vladimir Ilyich Lenin and find the equivalents on every detail to the writings and actions of Khomeini.

Like the communists saw the socialists, the Islamists see those closest to them -- the majority of conservative-traditionalist Muslims -- as one of their main enemies. Every effort had to be made to win over that group, from violence to persuasion, the use of front groups, and so on. The communists were stronger on party structure and hierarchical loyalty, two shortcomings from which the Islamists would suffer a great deal.

From the standpoint of the Iranian Islamists, like the Russian Bolsheviks, once they seized power the emphasis was put on the Tehran equivalent of what Joseph Stalin called "socialism in one country." In other words, they wanted to spread the revolution internationally but did not want to risk the survival of the Islamist regime in Iran by engaging in aggression that was too open. No matter how wild their rhetoric, Iranian leaders were more cautious in practice. But they also set up puppet and client groups which they felt would support Iranian interests. The Iranian infiltration into Iraq today -- using agents, money, and propaganda -- is very parallel to historical Soviet strategies for taking over countries.

As noted above, though, direct Iranian influence, organizational or ideological, is limited when it comes to the Sunni world. A range of local groups have sprang up, often building on their own ideological influences. Obviously these generalizations and parallels are imperfect but to make things simple, most of these groups can be divided into the Muslim Brotherhood and the al-Qaida (or jihadist) trends.

The Jihadists are the Trotskyists of the Islamist spectrum, very active but smaller in numbers, advocating permanent revolution and disdaining compromise or tactical maneuvers. For example, the Jihadists reject elections as an unwarranted human usurping of God's prerogatives and focus almost completely on armed struggle.

The Muslim Brotherhood in its various manifestations (including Hamas and Hezbollah) use terrorism but also put more emphasis on political organizing. They are happy to participate in elections once they have come to understand that they can actually win them.

Finally, this type of Islamism/communism parallel explains a lot of the appeal of radical Islamism to Western atheistic, hedonistic, leftists. Marxism-Leninism is dead and the proletariat is often conservative or at least is seeking to enjoy middle class lifestyles. Where are the foot soldiers of the revolution going to come from, the "masses" who will bring down the power structure the Western left hates?

Here is where the Islamists come in, real revolutionaries willing to put their lives on the line who take up the gun and have an ideology they believe in passionately. Sure, it's true that they believe the opposite of what their Western groupies think and indeed those leftists would be the first ones to be "put up against the wall" by Islamist terrorists. Unfortunately, though, it isn't the first time the Western far left revered totalitarians.

At any rate, it seems as if Islamism is the 21st century's equivalent of Marxism-Leninism in the 20th century. Politics makes strange brain-fellows.

(Barry Rubin is Director of the Global Research in International Affairs (GLORIA) Center, Interdisciplinary Center University.)