The United Stated had declared war on Germany in 1914, and during 1918 WWI was on its course.
Most of America's brewers were German natives that arrived in the U.S during the 19th century. Many of these breweries carried the names of these German families.
This particular poster mixes both WWI and German brewers who lived in the U.S. This poster asks the question "Will you back me or back booze?" and there's an image of a soldier in the trenches. We can assume that this poster is trying to ask people not to back Germans living in the country because the United States were at war with Germany, and given that many Germans living in the U.S brewed beer, the poster is using the word "booze" to refer to Germans.
