Terrorist attacks in Brussels

Because the Crusades were in response to muslim conquest. The Crusaders weren't ISIS, they were fighting ISIS. By the time the Crusades even started, muslim armies had conquered two-thirds of "christian" territories.

And the Conquistadores in Central America were in response to Aztec violence?

Every religion that views itself as the one and only Truth, and commands its believers to spread the word, is liable to be used as a tool by powerhungry or greedy leaders, to get their people to fight and die "in the name of God."

The average Muslim, like the average Christian, Jew, or Buddhist, would much rather just live their own lives, and stay out of trouble. This includes giving whatever answer they think the people in power want to hear, whenever asked for their opinion.

Iran, up until the 1970's, was a modern, "enlightened" country. It had high technology and good universities, and a largely "secular" population - they were Muslim in the same way most Americans are Christian: not fanatic, not letting religion get in the way of anything they actually wanted to do.
Unfortunately, the Shah was a despot, and the people didn't like how he hoarded so much of the wealth of the country, and used secret police and similar totalitarian tactics to continue to rule. So, the religious leaders used this point to work their way to power, overthrow the Shah, and with him all semblance of "corrupt" Western civilization.
They still keep their science and technology up, though, which makes them far more dangerous than such nuts as Ghaddafi of Lybia, who was totally dependent on the Russians.
Anyway, the average Iranian doesn't care one way or the other about the American or Zionist Devils their leaders keep talking about. The average Iranian wouldn't dare be caught saying so, however, because they value their life and semblance of freedom.

Trying to declare war on 1.5 billion people is not the way to go. What are you going to do? Round up every Muslim in every country where they're not the majority, and deport them? And then carpet-bomb (or nuke) all of North Africa, most of the Middle East, and large parts of Asia?
Do you think they will take that lying down? If you try to do this, you will get every single Muslim hating you and fighting you, because they will be fighting for their lives. And since there is no way to find them all (most Western countries don't keep any record of anyone's religion. And are you going to actually strip people of their citizenship based solely on what you think their religion is?), you'll only make the problem worse.