Sweden was officially neutral during World War I, although, under German pressure, they did take steps which were detrimental to the Allied powers including mining the Øresund channel, thus closing it to Allied shipping, and allowing the Germans to use Swedish facilities and the Swedish cipher to transmit secret messages to their overseas embassies. Sweden also allowed volunteers to fight for the White Guards alongside the Germans against the Red Guards and Russians in the Finnish Civil War, and briefly occupied the Åland Islands in co-operation with Germany.