20
Lesson 20 of 84 · Westward Expansion
Farming Technology on the Frontier
World War I (1914–1918) was one of the deadliest conflicts of its time. New weapons — machine guns, tanks, and poison gas — caused devastating casualties across Europe.
Key Facts
1
Timelines show events in order.
2
Cause and effect explain why things happen.
3
Oral histories preserve community stories.
Check Your Understanding
Question 1
1 of 2Which is a secondary source?