The best answer will be poison gas, machine gun, and submarines.
A letter written during the time a historical event occurred would be considered a primary source, as opposed to secondary source, since it gives the point of view of someone who lived through the event in. I think
Answer:
A Biography of a public figure, a report issued by a government agency, and a blog run by a college student.
Explanation:
A pamphelt from an activist group wouldn't help, as it's a pamphlet which have very little information for a essay. A website by a reputable newspaper would be good, however, most newspaper websites catch within new things, not as much historical stuff.
No European nation built an "empire" by its proper definition, but Great Britain was the first to exert major power over the continent having the 13 colonies under their control for some time. Spain could also be seen as a major player as well as they owned much of the Midwest territory that was eventually bought by Thomas Jefferson that came to be known as the Louisiana purchase.
