Short answers, pulled from the story.
Two consuls were elected each year to head the state government. These leaders received a consular army and an area in which to campaign during the early to mid-Republic.
Gaius Marius and Sulla changed how the army functioned forever during the 1st century BC. This shift created a powerful tool for personal power grabs and led to civil wars between rivals seeking total dominance.
The Roman army dominated much of the land surrounding the Mediterranean Sea at the Empire's height. Campaigns extended from Italy all the way to Britannia and Asia Minor while trade routes opened up as new provinces were added.
Naval battles were largely less important compared to land campaigns because land battles formed the core of military history over roughly thirteen centuries. Notable exceptions occurred only during the First Punic War when fleets clashed heavily.
Frequent changes in leadership destabilized the late empire significantly during the period known as the Crisis of the Third Century. Emperors were often killed by their own troops seeking better rewards while conspiracies formed within the ranks to overthrow existing rulers.