Large seaworthy vessels first appeared in the Mediterranean Sea, but of course it should be noted that they also appeared elsewhere in the world as well.
The reason why Great Britain and France declared war on Germany at the beginning of the second World war was because A - The invasion of Poland.
When Germany invaded Poland this was the beginning of what would later be called the second World war. A tense atmosphere was brewing up in Europe for a few years since then already because when Hitler's party took charge of Germany, it increasingly militarized the country and was preparing it for war.
During and after WWI, America experienced rapid growth due to the fact that pretty much the rest of the western civilization was demolished. American businesses were selling to Europe, and obviously earned a lot of profit. There was a lot of opportunity, and people wanted to spend their extra money on fun things. Advertisers and Marketers capitalized on that, and started a shift into more consumeristic thinking. With the advent of electricity, tons of home appliances were created and marketed, emphasizing the "buy now and pay later" approach. With all of the kitchen appliances, the era of domesticity ended, and women were free to go do other things such as have jobs or pursue fun activities.
The economy thrived, money was created out of nothing (compound interest, etc), which lead to a bubble and eventual collapse in 1929.
Hope this helps! :)
