Existentialism can basically be summed up by the statement: "existence precedes essence." If you don't have a background in philosophy, this may not make too much sense to you, but I'll try to give a bit of insight. There were two individuals that are widely given credit for starting the existentialist movement: Søren Kierkegaard and Friedrich Nietzsche. Now Kierkegaard was a Christian, and thus started a following of Christian existentialists. Nietzsche was anything but a Christian, widely known for his comment "God is dead."
To further describe the "existence precedes essence" bit, perhaps an analogy is in order. Philosophers before Nietzsche and Kierkegaard believed that the essense of a human, or its purpose or meaning, preceded the existence of a human being. This is largely an Aristotelian idea, and the existentialists completely reversed this idea. Thier ideas state that the human itself defines its purpose or meaning through existing.
As far as imperialism and indochina, I better not fool you with a false answer, because I am not well versed on those subjects!
minor in philosophy