After the repeated sinking of American ships, the President declares war. Historic film footage shows some of the first naval battles.

Learn about the use of poisonous gas during WWI.

After two years of trench warfare, the Allies attempt to break through German lines on the Western Front. The ensuing battle will last for months and result in more than one million casualties.

In the years leading up to WWI, a series of agreements between the powers of Europe helped determine where and when battlelines were drawn.