Final answer:
The United States mobilized for WWI through propaganda that emphasized nationalism and patriotism, particularly among immigrants, to build support for the war effort.
Step-by-step explanation:
The United States mobilized for World War I using a variety of strategies that included securing manpower, financing, and supplies, as well as swaying public opinion in favor of the war effort. One of the measures adopted was the government's pro-war propaganda campaign, aimed at fostering national unity among a diverse population with varied allegiances owing to their heritage. The propaganda posters, like the one described above, played a crucial role in appealing to American patriotism among immigrants by emphasizing their debt to America and suppressing any dissent towards the U.S. involvement in the war.
The posters reinforced the idea of 'American First, American Above All' which was critical to build a unified support for the war effort regardless of an individual's origins. They clearly illustrated the changes in American character, showcasing a shift from a nation of diverse immigrants to a more homogenous and patriotic society that was willing to set aside ethnic differences and collectively support the country in times of global conflict. Such propaganda also included promoting the purchase of war bonds which financially supported the war effort and reinforced the moral imperative to back the American troops fighting overseas.