Apart from a few thousand years of History that can be debated...

Basically, you'd debate about which key events led up to a specific moment, or which were the most important. For example:

- Did Serbian nationalism play the biggest role in causing World War I?
- During his reign, did Tsar Alexander II do more to secure Tsarism and autocracy, or more to damage it?
- To what extent was money from big business the key factor in the Nazi rise to power, 1919 - 1933?