Because once an indentured servants time is up, you have to let them live their own life, but if you have a slave, you literally own them and keep them until they die if you choose to do so.
His invention allowed people to build factories far from rivers
Each country had a different economic system (capitalism in the USA and communism in the SU), which caused competition.
This competition led to military competition and a nuclear arms race, which was called the Cold War. The nuclear arms race became the greatest point of tension.
