The Logo for Lesson 18 is a "Calavera"

Question 18 SA-4. Here is the text reference (p. 171/3):

In one of the more bizarre incidents of the Mexican Revolution, in March 1916 Pancho Villa ordered his men to cross the border and attack the small town of Columbus, New Mexico, killing several Americans and destroying parts of the town and the small military outpost. The reasons for the raid were unclear, but apparently Villa hoped that his brazen act and the expected US response would appeal to Mexican nationalism and result in greater support for his group. The raid was, technically, the last time the continental United States was invaded by a hostile foreign force. U.S. public opinion was outraged, and President Woodrow Wilson ordered General John Pershing to mount a Punitive Expedition to catch Villa. This proved to be a frustrating and difficult task, and after a year of marching around the deserts of northern Mexico in pursuit of Villa the Expedition gave up and returned to the United States.