As a teacher and student of history, I can't help but reflect on this one truism: empires fall soon after they stop seeking to expand. We have the technology to put humankind farther than we have and we have the knowledge needed to push the technology so that we can go even beyond that. As we become more isolationist and tighten our borders we will find that rather than protecting ourselves, we are tying our own noose.
Zheng Hu's massive fleets of ships were left to rot and now the Ming are mostly known for their vases. It was the pitifully small countries of Portugal and the newly formed Spain that sent our their tiny exploring vessels and benefited from their new connections. And when they decided to try pull back their exploration, it was the Dutch, French, and British who took their place.
Again and again, it is the initiative to explore that creates new empires while old empires rot on their laurels. Our frontiers are daunting, but no less daunting than the frontiers were to the explorers of old. We must explore, and expand, or lose ourselves entirely.