I’d definitely start with Ancient Greece as it preceded Romans and Romans had to wage war agains Greek states in the beginning to become the real deal. Perhaps

I’d definitely start with Ancient Greece as it preceded Romans and Romans had to wage war agains Greek states in the beginning to become the real deal. Perhaps even start with Alexander the Great and his conquest. I would then look into Carthaginian wars, famous general Hannibal and his fate. This was a crucial period when Rome was competing with Carthage for the top spot and at times it looked quite dire for Rome as Carthage had an upper hand. Next interesting period would be Julius Ceazar and especially what happened after his death when his generals were battling to inherit his empire which eventually led to the split into eastern and western Romans. This split later continued when Christianity became mainstream religion and lead to Byzantine Empire. You can look into the sack of Constantinople and wonder what would happen if Western Christians didn’t sack and destroy Eastern Christians allowing Islam to take over the region. And then we come to the German rise as German lands become the heart of the indirect continuation of the Roman Empire in the form of Holy Roman Empire.