<p>For most defense contractors, you probably will need a security clearance. It is one reason why so many former/retired military work for defense contractors. They usually have a clearance and they can go to work immediately. </p>
<p>Clearances can take a very long time to process depending on the level (secret, Top Secret) and how much they have to investigate (if you have traveled overseas extensively, family in other countries etc.). The adjudication process can take up to a year. </p>
<p>If you are interested in working for a defense company, I would suggest you become a citizen as it will make the process easier for you.</p>