The Lusitania was a steamship owned by the british that was being used secretly for war in 1915. It was disguised as a passenger ship but was carrying out weapons and supplies for war from the United states to Europe. The Germans knew that british were using the Steamship and they declared the water surrounding as a war zone and the Germans used their U-boat to sink the ship in May 7 1915 and claimed self defense for it.
On February 15 1898 the USS maine was a ship that was in Cuba's Harbor that was blown up by a mine but spain was blamed for it and America declared war. USS maine was in Cuba's Harbor just to warn spain about the people that United States had in spain.
On June 28, 1914 Archduke Franz Ferdinand was in Bosnia when he was in his car going to the capital city of Sarajevo when a serbian terrorists threw a bomb in their car and drove by and shot both Archduke and his wife. Within days Germany declared War on Serbia's ally which was Russia, then they invaded France via Belgium and Britain then declared war on Germany.