Because of our historical role and that of some allies, the ability for us to take some damage and still stand (Something as small as the GSG9 in Somalia failing in 78 would have sunk Schmidt politically), our political clout, economic might, military and intelligence capabilities, our ability to work with any party (some of our allies are like water and oil, i.e. Japan/S. Korea, Turkey/Greece, after WWII Germany/France for a while......... Besides, for those who are fighting for ideological reasons, the US is also symbolic, a representation of globalization, Western interests, the Judea Christian world and the expanse of this Western liberal culture that is even creeping into the Muslim/Arab world. They address us in their hate speeches in Iran. Osama addresses us in his speeches. Saddam addressed us in his speeches. Qaddafi addresses us in his speeches............ We are the leader when it comes to the “real threats” out there and our enemies see it that way too.
See, that is a problem. Globalization is simply a threat in itself. It is right that the US represents that, but therefore it represents a vital threat to the existing nations, more damaging than terrorism. It is even a threat to the United States, not that we as a nation recognize it.
I would support expansion of Christian culture, but expansion of Western liberal culture is also not a welcome thing in most of the world, and for a good reason. When you say "Western liberal culture" you probably mean tolerance of opposite views, pluralistic society, representative government. That is what it means in the Christian context; as we lose Christianity as the rule of life, these become empty slogans; to non-Christian nations they ar simply meaningless. To the world outside of the US and Europe, western liberalism means destruction of traditional way of life, introduction of public vice, civil war, corrupt factional governments serving foreign interests. Naturally, the nations of the world resist that.
If we simply defended our turf, like retaliating for attacks on us, everyone would understand. If we had a moral society where people had superior ethical values (for example, had stable families raising chaste children), and wanted to teach others how to achieve that, again many would understand. But we took on that role of a cultural and political crusader, whereas secular liberal culture that we have -- as opposed to Christian culture -- is not attractive to many for many good reasons.