A) Vikings were the first Europeans to encounter North Americans. Although it is much less historically popular, the Vikings were the first Europeans to have interactions with the indigenous peoples of North America. The Vikings came to North America to pillage and raid the villages, but some were also involved in trade.
The exact cause of the war
The economy was not doing well, soldiers were coming home with horrible PTSD and after-shock, the great depression was going on, etc. :)
One approach was that of President Roosevelt who saw American imperialism as a form of improving the society. If the United States were to conquer others, they could then implement their progressive ways and democratize societies. Other progressives were bothered by this and didn't want military interventions because they knew what horrors are caused by wars. Either way, they were against isolationism and believed the US should deal with foreign politics as much as possible.