George Santayana said, "Those who cannot remember the past are doomed to repeat it." Modern world History is the class where you learn that "past." You learn what mistakes lead to disasters, you learn what circumstances lead to breakthroughs. You learn what worked and what didn't.
Once you know the past, you "remember the past," and so you aren't doomed to repeat it. You can realize when history is about "repeat itself," and prevent it from happening by doing something differently.
The current flap over Syria is a key example. President Obama put down a "red line," and Bashar Assad had the audacity to step over that line. Now he has to decide whether to back that line with real consequences or empty threats. In 1994, the UN put a similar Red Line in Bosnia, and backed it with empty threats. I remember that the result was the Massacre at Srebrenica. Which is why I support imposing at least symbolic consequences.
Modern world History is where you will learn about the same, so that in the future, you too know when to hold the line; and so much more.