Most likely than not you cannot destroy a naval invasion of that scale. The Scipii are the strongest naval power around, and they will launch massive navies escorting equally massive armies against you. The Scipii have most of the islands west of Italy, the whole of North Africa, and are likely to be fighting Egypt or what Eastern power has conquered those Egyptian lands. They should also have one or two islands near Greece. It may or may not have expanded to Turkey, or even to Asia Minor. The Brutii empire is in Southern Italy as well as Greece. If you didn’t, then defeat is already certain. If you have built a strong navy, defence of your Western European provinces shouldn’t be a problem. The Julii have the upper-hand in this war. The Senate often disowns one faction more than once. In rapid succession, the other Roman families, including your own, will get disowned. The least-loved Roman family will get disowned by the Senate, triggering the Civil War. From the start, your faithful allies the Senate, Scipii, and Brutii would have expanded and made their own empires (the Senate, however, is stuck in Rome.) You weren’t allowed to break alliances or do any aggressive maneouvers on your Roman allies’ territory. The Roman Civil War is a game-initiated event. As you finish off your foes, the Julii empire is being formed. Once Spain is conquered, and your armies are out of the Pre-Marian era, you can start murdering the Britons, withdrawing your support for the Gauls, backstab the Germans… But one at a time, of course. Even the elite Spanish infantry unit, Bull Warriors, don’t stand up to Early Legionary Cohorts… If you use the Legionaries’ superior defence stats properly. They have bad unit rosters, and by now you should have had the Marian Reforms, so Roman Cavalry or Legionary Cavalry should handle those Longshields well. You should make the Gauls your protectorate by this point in time. The Britons often end up fighting the Germans, so leave the Gauls with a province or two to protect yourself from the Britons. Get German support, exchange trade rights, maybe even an alliance, depending on the Senate’s policy towards them. Attacking the Germans first leaves you unprepared for their powerful charges, and allows the Gauls and Thracians and Dacians to flank you.

Rome total war protectorate