Please, do not forgot that declarations are both internal (for French people) and external (allied countries/politicals, opponents).

The facts that some French SOF fails to rescue the hostage simply means that intelligence/S2 wasn't able to collect enough data.

The hostage suffers heart disease and had no medicine from weeks : no one can tell (until now) if he was still alive when the raid was launched.

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Regarding French laws, WAR can only be declared against a country.