Elon Musk "secretly" disrupted a Ukrainian sneak attack on a Russian naval fleet by turning off Starlink's satellite network near the Crimean coast last year.
It isn't legally treason or insurrection. I'm not sure what the legal term would be for what he did, but it does open him up directly for an act of retorsion from either the US or Ukraine, so it is illegal under international law.
The fun thing about an act of retorsion is that it doesn't have to follow the "proportionate response" rule, so Ukraine can go as nuclear as they want against Musk, and The Hague will support them.