Ever since the indictments of Slobodan Milosevic and Charles Taylor, it’s become a settled legal principle that leaders can be guilty of crimes committed outside their own countries, and which they have no personal knowledge of. It’s known as being a member of a “joint criminal enterprise”. and it requires just proof of contacts with people in the country concerned who are alleged to have committed crimes, and at least some influence over them. Ignorance is not a defence if you “knew or should have known” that crimes would be committed. This also extends to private individuals: a number of Dutch businessmen were prosecuted for supplying chemicals to the Saddam Hussein regime, which were later used against the Kurds.
Which brings us to Syria. Whatever western governments may say, the reality is that they know, or should know, that arms they may supply to rebel forces could wind up anywhere, and be used for anything, including illegal acts. And moreover they are supplying these arms precisely in order to have influence over certain groups.
If I were President Hollande, I’d make sure I had a good lawyer lined up.