You're thinking of an empire in the 18th and 19th-century sense of the word – a sense that died its last official breath after WWII, when Britain released the last of its official colonies. In that era, when the nation-state was the ultimate expression of power, a colony flew the colonizer's flag, spoke its language, had the colonizer's religion imposed upon it. Going back into the heyday of colonialism, conquest was government-centric; national glory was the cause.
With the rise of international business, however, the nation-state itself has been supplanted by the multinational corporation. They do not work for the glory of the nation, but for their own glory. They do not respect the laws of the nation, and do not obey except where those laws are convenient or enforceable. In short, the heyday of the nation-state is over.
Let it not be said that the nation-state is dead, though. We're still in the centuries-long transition between forms of cultural organization, so while governments are the only ones permitted to hold the weapons (this, too is changing and will continue to change over our lifetimes), the multinationals' interests dictate where those weapons are pointed and when.
This is why the United States has military presence in over a hundred countries in a time of peace. These are the agents of modern colonialism.
This is why there are terrorist attacks against our troops and our cities and citizens. Not because they hate our freedoms, but because we are camped out, toting guns, on their land, and have been for a hundred years now.
