The Lusitania is a British ocean liner that carries passengers between the United Kingdom and the United States. It is currently crossing the Atlantic from Liverpool to New York, but its owners are already taking bookings for the return voyage back to Britain.
Germany has declared the seas around Britain and Ireland to be a war zone where British ships are liable to be sunk without warning. British naval dominance has Germany’s main fleet confined to port, but its U-boats are on the prowl, attacking Allied shipping where they can.
As a British ship, even a civilian one, the Lusitania is to the Germans a legitimate target for their submarines. Nevertheless, they fear the consequences on American opinion if the ship is sunk with American passengers onboard. Accordingly, the German embassy in Washington DC places advertisements in American newspapers, warning against travel on ships flying the flag of Britain or any of its allies. Such ships are liable to destruction and anyone travelling on them does so at their own risk.
image source (Wikipedia)