<p>actually permanent residents (Green Card) can obtain security clearance. I know a person who immigrated here on a student visa that currently works at boeing on top secret military aircraft project. If you are an international student, first try to get H1 B/work visa, then after working for a number of years, apply for Green Card. I think you can get US citizenship after being on Green Card for a number of years as well.
Or you can get a Green Card through marriage to a US citizen but I am not sure how it will look upon your employment status.</p>