The Denver Broncos are my favorite American football team.
I do not know much about them. I don't really pay attention to football.
Well they first played in the AFL in 1960 and then they joined the NFL in 1970, when they merged.
Why did they merge?
They merged to boost the popularity of the sport in the United States.
Okay. That makes sense. Did the 2 leagues compete or were they totally separate? 
I'm not sure, but the Broncos weren't that competitive during their 10 years in the AFL.
I wonder why they weren't.
The first time they advanced to the playoffs was in 1977.
Were they successful in the playoffs?