Subsec. (a)(3). Pub. L. 99–653, § 18(d), as amended by Pub. L. 100–525, § 8(m)(2), [ changed from ] for “unless, prior to such entry or service, such entry or service is specifically authorized in writing by the Secretary of State and the Secretary of Defense
That's what I was remembering. It used to require a bunch of permission letters to do this, and now it's just "oh, hey, you still want to be an American? Coo'."
From the Cornell link in that page:
That's what I was remembering. It used to require a bunch of permission letters to do this, and now it's just "oh, hey, you still want to be an American? Coo'."