The United States first began developing nuclear weapons in 1939, during World War II, under the order of President Franklin Roosevelt. Motivation was the fear that US was engaged in a race with Nazi Germany to develop nuclear weapon. This project is best known as Manhattan Project.
Hitler emphasized on taking land fast, so he created the Blitzkrieg, or lighting war. It relied on mobile units, or a mobilization army and limited artillery to create confusion and disrupt enemy front lights. This war strategy saved a lot of lives and was considered genius. The tactic worked extremely well on Poland, and eventually broke into France.
The era of good feelings was from 1815 to 1825 because the Napoleonic wars ended
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law passing to other states
