But hasn't Islam always been a political ideology that is expansionist and violent?

Islam, like Christianity, is a web of doctrine and practice that has been interpreted many ways by people over time--including different ways by different people at the same point in time.

I'll discuss several facets of early Islam that show how simple understandings of peaceful versus violent, tolerant versus intolerant are matters of individuals and group rather than inherent to a belief system. Hopefully one of our amazing experts in more modern Middle Eastern topics will be along to discuss the 20th century situation.

First, let's talk about the initial expansion of "Islam" out of Arabia. Less than a century after the hajj, Umayyad forces have swept across North Africa to establish a western nexus on the edge of the world, in Iberia! Make no mistake, this is hard-fought military conquest; we know about pockets of military resistance operating even centuries later.

But the question here is, to what extent can we call this the expansion of Islam? It is one thing to point to verses in the Quran about fighting unbelievers until they submit. It is another to recognize, as many modern scholars posit, that there were significant numbers of Jews and Christians fighting in the Arab armies. Baladhuri mentions that members of the two religions in Samaria served as spies during the Arab conquest and were generously rewarded.

When we look at Umayyad policy, additionally, we don't see widespread mass conversion or efforts at forced conversion. It is apparent that Christians, Jews and also Zoroastrians (the ones who did not convert for political reasons in Persia) adopted Arabic language and some Arab customs early and easily. Actual religious conversion on a large scale occurred only over the course of centuries--as individuals and families found it more advantageous to live as Muslims than as dhimmis.

There are a couple cases of attempts at forced conversion before 1000 (in the 11th-12th century, Christians and Muslims alike start to take a harder-line stance on the flexibility of religious belief, although there is not nearly as much comparative research on the simultaneous developments as there ought to be). What's unique about these? The perpetrators are universally denounced by other Muslims. They are labeled rebels or bandits, and Muslim chroniclers note the failure and cessation of their forced conversion campaigns as, in the words of Abu al-Fath, "relief came from God, and he sent mercy because he is merciful and benevolent."

Second, let's talk about jihad. It's hard to think of a word that carries more ideological and polemical weight in modern political discourse. Among Islamic scholars in the early centuries, it was almost as contentious. Mottahedeh and Laiou have shown how, in the 7th and 8th centuries, different legal scholars constructed interpretations on whether jihad was only legitimate if it were defensive warfare, or whether it also encompassed offensive warfare. The point is the multiplicity of views.

By the 8th and definitely the 9th century, the Byzantine Empire starts to win some minor and significant victories against Arab armies, halting the expansion northward. In an atmosphere stripped of the apocalyptic expectation of conquest and triumph that had fueled the "offensive warfare" definition of jihad, scholars worked to forge new understandings.

An important line of interpretation emerged that had a lot to do with preserving the trade and diplomatic situation across the Mediterranean. Scholars divided the world into dar al-Islam and dar al-harb, the house of Islam and the house of war. Although jihad still needed to be fought, the terrain for the battle was always "out there" in the world of war. Jihad, following this line of interpretation, became spiritual warfare on the part of pious Muslims safely in the dar al-Islam.

This division of the world held great sway. One of the fascinating things Kathryn Miller uncovered in her exploration of the Arabic archives from the 15th century Ebro Valley (Spain) is that imams from North Africa wrote furiously to Spanish Muslims, utterly flabbergasted that they'd remain in the Christian world and begging/exhorting/ordering them to return to dar al-Islam. Not to go on the offensive, not to convert. To return to the world of Islam.

I am a medievalist and I shall stop my story here. But I think there is ample evidence to show the complexity of "Islam": not an unchanging ideology but a social and theological religion that people interpreted in many ways at the same time and over the course of time.

The are very good answers here on the Mediaeval history of Islam and it's doctrine, I would like to add a little more about the creation of modern Islamism.

Islamism as a movement began in the 19th century and early 20th century. The primary cause was the decline of the Ottoman Empire, and it's dismemberment by European powers. Also important was British rule over India, but I will not talk about that because it's outside of my expertise.

By the late 19th century, Egypt, the heart of the Arab world, was ruled by a puppet regime of the British, and parts of its territory directly occupied by the British to protect the Suez Canal. The Ottoman Empire was frequently forced to accept European diktats, and after the First World War was dismantled by European imperialists who divided it's territory amongst themselves. The Caliphate was dismantled by the Turkish Republic, and a 1000 year tradition of God's chosen ruler on Earth came to an end. In explaining the calamity of the collapse of the Islamic world view (House of Peace led by the Caliph, House of War of barbarians outside the law), and explaining the evident superiority of non-Islamic Europeans, a movement led by Hassan al-Banna cast the blame on the decadence of Muslims, and the way they did not follow the purity of the early Islamic rulers. Thus, Muslims must cast aside the traps of Western materialism, and remake themselves in piety. Al-Banna's movement is what became the Muslim Brotherhood.

So, Islamism is influenced by the West, but more in terms of the challenge the West's development poses to Islam's ability to explain the world