| A | B |
|---|
| Allies | Britain, France, and Russia |
| Central Powers | Germany, Austria-Hungary, and Turkey (Ottoman Empire) |
| Submarines | Underwater ships (U-boats) that shoot torpedoes to sink ships without warning. |
| Western Front | The war in France and Belgium |
| Eastern Front | The war in Russia and Austria-Hungary |
| Trenches | Long ditches dug to protect soldiers against attack. |
| Naval mines | Floating bombs that detonate when ships bump into them. |
| War zone | The zone surrounding Britain which Germany declared it would sink ships. |
| Kaiser | The German head of state (from "Caesar") |
| Lusitania | British passenger ship sunk by a German submarine. |
| Sussex | French passenger ship sunk by a German submarine. |
| Sussex Pledge | Assurance by Germany that it would not sink merchant ships without warning and would not sink passenger ships at all. |
| Unrestricted submarine warfare | German decision to sink all ships in the war zone around Britain. |
| Zimmermann telegram | Note sent by a German leader proposing that Mexico ally with Germany against the United States. In return, Germany would help Mexico reclaim land in the southwestern U.S. |
