Unfortunately, you didn't share any following reasons so that someony could choose the correct one, but I am pretty sure that I can help you and name the main factor. Definitely, the point which caused a decrease in the size of the armed forces at the end of the cold war is that the US promised to decrease its armed forces as part of the peacetime agreement. I bet it will help you!
The secret service, which is a federal policing agency that was part of the department of Treasury. At least until 2003, when is became the department of Homeland Security.
It is considered the first decade of the modern era because it made a big change move from farming cultures toward capitalism, industrialization, secularization, rationalization, and the nation-state.
Hope I helped :)
