Charts and graphs created by military or other government sources can tell historians what information officials had at the time when they were making critical decisions. This chart from 1940 in the example below shows U-boat activity in conjunction with the number of merchant ships sunk and the number of U-boats destroyed.
Source: CHAR 20/135/19
Description: Map charting the supply routes to Britain.
This source shows the number of U-boats (German submarines) operating, the number of U-boats sunk, and the number of merchant ships sunk from July to December 1940.
German submarines were an important component of the Axis strategy. By destroying merchant ships and posing a threat to military vessels, U-boats disrupted supply lines and made naval missions in the Atlantic risky. Supplies of food and essential war materials from the rest of the world were essential in order to keep Britain’s war effort alive.
For more information about U-boats, explore our investigation on the Battle of the Atlantic.
Choose from the types of primary source below to explore detailed examples that show useful ways to interpret each category, with handy tips and questions to ask yourself: