It’s very much legal in wartime, for exactly those sorts of purposes. Though I’m not sure we’ve tested the legality of it in this modern world where nobody actually formally declares war anymore—I don’t think it’s been attempted.

(Please don’t flame thinking I’m hardcore in support of a particular side in this war due to this post—you’ve very likely gotten the wrong impression. I’m commenting only on the narrow point that the US in fact can censor, including with prior restraint in certain circumstances, during war.)