Despite the harsh winter of 1916-17, the North Sea and Atlantic Ocean saw a series of decisive naval battles. In an attempt to choke off the transatlantic supply lines of the Western Allies, German U-boat submarines were dispatched to torpedo merchant vessels regardless of their nationality. Although Britain ran short on matériel for a time, the U-boat attacks on South American-flagged merchant vessels finally provoked the countries of South America -- first Brazil and Chile, then Argentina, Uruguay, and Venezuela -- into declaring war against Germany in January and February of 1917.