I believe the answer is the Industrial Revolution
Answer:
=> The demographic future for the U.S. and the world looks very different than the recent past in key respects. Growth from 1950 to 2010 was rapid—the global population nearly tripled, and the U.S. population doubled. However, population growth from 2010 to 2050 is projected to be significantly slower and is expected to tilt strongly to the oldest age groups, both globally and in the U.S.
=> Aging is not exactly news—the U.S. and global populations also turned older from 1950 to 2010. But future prospects for aging have garnered more attention because, unlike in the past, younger populations, those of children and those of middle-age adults, are at near standstill. Thus, the social and economic effects of aging are likely to be felt more acutely in the future.
Explanation:

Answer:
On June 10,1940 US president Franklin D Roosevelt gave his famous speech "Stab in the Back" at University of Virginia.
On the speech He mentioned that the hand that held the dagger has struck into it's neighbor's back. Here he referred Italy and Mussolini as the hand which held dagger and France was the neighbor.
Because on June 10,1940 Italy declared war against France and Britain and joined to Germany on axis power. Mussolini said Italy was joining war against France and Britain as for keeping promise to Germany.
Roosevelt mentioned this incident for in his speech as the hand held dagger has struck into France's back.
Moreover, in this speech, He made a proclamation about removing America's neutral position in war and soon declared the Mediterranean see as a combat zone.
Explanation:
<span>John C. Calhoun
was the US South Senator who believed that slavery was necessary. He was a
political theorist that believed in republicanism, which also included the
approval of slavery. He defended slavery as a “positive good” and as a result
Southern minority threats escalated. </span>
Immediately after World War I, the conditions in Japan were different in Germany because <span>Japan emerged as a world power, but Germany was weak and humiliated.</span>