Answer:
Both were emerging as economic and military potencies in the early 20th century.
Step-by-step explanation:
Let's describe each country separately. This way will be easier to understand their similarities.
THE UNITED STATES
In the early 20th Century, America was a rising economy. For the most part, the Industrial Revolution, in many cases caused by the use of oil, was profitable and offered a range of changes. We can list several of them, but let's highlight the build of a railroad, the high development of cities, and the formation of a military body as the most important. Plus, America begins to receive many immigrants, creating a multicultural society. The effects of the Industrial Revolution and the increase of economical profit led to the formation of a new lifestyle, characterized by the opulence and the wealthy of many families. What we call as the middle class was born in the early 20th century. From 1900 to 1930, America turns into an economic and military potency.
JAPAN
On the other hand, Japan faced a different approach in the 19th Century. In 1868, a coup d'etat led by the Japanese nobility put down the Tokugawa Shogunate (徳川幕府) putting an end into a long period of isolationism and xenophobia. The Tokugawa Shogunate ruled Japan for almost 200 years, delaying the country's political and military development. The coup was supported by America which makes the new emperor an ally. But now, Japan needed to seek to develop its army, it's economy and it's respect to the world. America helped Japan on the first efforts, especially economic and political help.