War and neutrality — War in general — Effects of outbreak of war — Trading with the enemy — Validity of contracts concluded with company of enemy nationality — France — German occupation and Vichy régime — Transaction authorized by de facto French authorities — French legislation of 1945 prohibiting trading with the enemy — Contract concluded between French and German companies in 1943 involving transfer of shares in Roumanian oil company — Whether valid — Test of whether transaction would appreciably assist enemy economically — The law of France