What were the three most important ideas in roman philosophy and law?

The Romans first employed a citizen-army and it became a great nation. Later it used a mercenary army, and the civilization collapsed.

The United States used to employ a citizen-army and became a great nation. Today in Iraq and Afghanistan we use a "volunteer" army of well-paid citizens and mercenaries (Xe), and ... (Some people say we can learn from history).

The Justinian Law Code provides the legal basis for about 75% of the countries of the world. Sure, they acknowledge the Little French General, but Justinian was more influential