The philosophy of Marxism believes that the forces of history make it inevitable that Socialism will sweep the globe and bring a worldwide workers' paradise into existence.
The religion of Islam believes that they have a divine mandate to conquer the world in God's name.
America is neither Islamic nor Socialist. We do not rule the world but we are the most influential country on Earth with our economic, cultural, military, and political influence reaching into virtually every country on earth.
To those who believe that either God or history has promised them world wide victory this is anathema.
The most powerful nation on earth has rejected socialism so the socialists hate us because their beliefs require that socialism dominate the Earth.
The most powerful nation on earth has rejected Islam so the Jihadists hate us because their beliefs require that the dominant power on Earth be Islam.
Our power is not acceptable to those who believe that power should be theirs alone. We stand as an iron wall against their dream of world wide victory. If we don't fall either into nothingness or else embrace their beliefs then their beliefs would be undermined.
9-11 happened because America's influence is more powerful than Islam's influence. It would have probably happened even if we didn't have a single soldier or ship in the Islamic world. It didn't happen because we were there but it happened because we exist. America, as it currently exists, does not fit into their dream of a worldwide Caliphate and that Caliphate will not exist until America is destroyed or conquered.
We must never let that happen.
Some think that the 9-11 hijackers simply got lucky and pulled off their insane dream. Maybe or maybe not. How long until they get lucky again and even more people die?
To many people there is only one evil on Earth and that is America. Any evil has to be blamed on us so 9-11 HAD to be an inside job. The unnatural relationship between Islam and Socialism is born mostly of a hatred for America and each group dreaming of the America they could possibly create if they had absolute power.
Very few deny that America is a flawed country, but as far as nations go, we are morally one of the best nations around. We are not perfect of course (no country is) but still a country to be proud of.
Update: thanks to Don's Mind for linking to me.