Joseph Conrad on "brute force"

This passage from The Heart of Darkness by Joseph Conrad captured my mind as I was reading.
They were conquerors [the character Marlow is speaking of the Romans in Britian], and for that you want only brute force - nothing to boast of, when you have it, since your strength is just an accident arising from the weakness of others. They grabbed what they could get for the sake of what was to be got. It was just robbery with violence, aggravated murder on a great scale, and men going at it blind - as is very proper for those who tackle a darkness. The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much. What redeems it is the idea only. An idea at the back of it; not a sentimental pretence but an idea; and an unselfish belief in the idea - something you can set up, and bow down before, and offer a sacrifice to . . . "

Here's the question - can belief in an idea be unselfish? Does the answer to that question depend on whether one shares that idea (and is willing to offer a sacrifice) only when others express a desire to share that belief or whether one thrusts that idea at others with the expectation that they too will be willing to offer a sacrifice to the idea?

If you think about what Conrad's character says through the lens of the war in the Middle East, does it give a different perspective?