It was on April 23, 1918, that the British cruiser "Vindictive" defended the operation of sinking two old cruisers filled with cement to block the submarine base at Zeebrugge. On May 10 the battered "Vindictive" herself was sunk to close the base at Ostend.
Spanish ships were continued to be sunk and on April 11, 1918, it was announced that Germany had begun a submarine blockade of Spanish ports as a result of a commercial treaty signed between Spain and the United States.
On the same day Uruguay asked the German Government, through the Swiss minister at Berlin, whether Germany considered that a state of war existed between the two countries. The inquiry was the result of the capture by a submarine of a Uruguayan military commission bound for France.
One of the largest German submarines appeared, on April 10, 1918, in the port of Monrovia, the capital of Liberia, on the west coast of Africa, after having seized the day before the small armed Liberian vessel President Grant. The crew were taken prisoners and the boat sunk. Liberia, it will be remembered, had declared war against Germany on August 4, 1917. The German commander dispatched by the Liberian crew an ultimatum to the Liberian Government, in which he threatened that, failing the dismantling of the wireless stations and the closing of the French cable, the town of Monrovia would be bombarded. The stations were accordingly closed, as the capital was under the fire of the German guns, but later the U-boat commander insisted upon their being destroyed. This the Liberian Government refused to do, and the German submarine thereupon bombarded Monrovia for over an hour, destroyed the stations, and inflicted some casualties. Fortunately for the town a steamer appeared at that moment. The submarine gave chase and did not return again.
The most important American losses in April, 1918, were the S. S. Lake Moor, manned by naval reserves and sunk in European waters on April 11, 1918, with a loss of 5 officers and 39 men; the Florence H., wrecked by an internal explosion while at anchor in a French port, with a loss of 29.
Neutral shipping continued to be a heavy loser. The Norwegian Government, for instance, announced that from the beginning of the war to the end of April, 1918, Norway's losses had reached the total of 755 vessels, aggregating 1,115,519 tons, and accompanied by the loss of 1,006 seamen, while 700 more out of additional 53 ships were missing.
On the other hand it was announced that during April, 1918, 12 German submarines had been captured or sunk in European waters by American and British destroyers.
On May 3, 1918, the Old Dominion liner Tyler was sunk off the coast of France with a loss of 11 men, including 5 naval gunners.