How did the Romans manage to effectively combine their whole empire into 1 unit?

The Romans effectively made their one empire by mainly one factor - war. Through war, the Romans conquered whole countries. They captured one country at a time, then made order. After that, they would settle the land, until the once-seperate countries became part of Rome. Then they went back to another war. And defeated another country. And then kept that one in line. The making of the Roman Empire was not overnight. It took years and years, but one by one, the Romans gained more people and more power. They gained treasures from the defeated countries. As they won, they gained people, soldiers and better chances of beating the other army. Because they waorked so hard and so steadily, they won. Slow and steady wins the race.