He shared a video of Russian troops committing crimes, in Ukraine. Which they are - both in that video and thousands of others besides. Truthful statements are not libel.
But they might impact national security.
Specifically, the First Amendment protection of free speech (and the identical rules incorporated against the states by the 14th Amedment) are why, among other impacts:
(1) In the US, falsity is an element of libel, rather than truth being a defense (or, in some foreign jurisdictions, not even necessarily being defense always.)
(2) In the US, libel against public figures (either in general, or limited purpose public figures within the scope in which they are public figures), requires the plaintiff to prove actual malice on top of the elements of libel that apply in other cases.
We can prove it false right now.
The US committed multiple crimes in Iraq and Afghanistan.
There, I said it.
If grandparent poster is correct, then I guess I'll be arrested here in America for committing the same crime.
We'll see.
Libel isn't free speech either.