Sources of images used for this section as they appear, top to bottom:

• Mustard Gas Burns, Wikimedia
• Council of Four Versailles, Capt. Jackson, US Army Signal Corps, Wikimedia
• Europe 1914, The National Archives UK
• U-25, Wikimedia
• Australian infantry small box respirators Ypres 1917, Captain Frank Hurley, Wikimedia
• Swiss Machine gun 1911, Hmaag, Wikimedia
• British Mark I male tank Somme 25 September 1916, Wikimedia