You start out with small tribes, scattered all over the world. Your resources are limited. Create armies and expand into city-states, grabbing high-value product. Convert this product into war materials.
As you conquer more territories you gain more resources. Claim countries and get bonuses. Build up your defenses. Protect your sea lanes with navies. Send out marines to claim new territories and islands.
As you grow, so do your opponents. Clashes are inevitable. Everyone wants the high-value product. Wars break out. Missiles fly.
When you reach super-power status, with an inventory of nuclear and conventional missiles, natural barriers are no longer a problem. You have the resources to launch large-scale attacks.
There is no randomness to a battle. As in chess, the attacker always wins. But your opponent may see a gap in your defenses and turn the tables on you. You could win a battle but lose the war.
Should you build your defenses? Or attack your opponent when he's weak? Remember, he who strikes first has the advantage.
Grow your empire and dominate the world!