If you are going to attack over the sea, you need to do one of two things. If you are close enough, say in the East Indies, Nile Delta, British Isles, or Warring States maps, build sufficient forts so that your borders extend onto the island that you are going to invade, or to a nearby island, and then build a barracks, auto plant, and factory, plus airbases there. You have to have civilian units to do this.
The other way is to make sure that you bring civilians along with you and as soon as you capture a city, build a fort to hold the territory for your side, and then build your military production buildings. Prior to launching your attack, it is a good idea to build the Versailles Wonder, and include supply wagons with your army. This way, you can heal your troops on enemy territory. If you simply attack with a large army, you are not going to succeed in the long run.
Also, remember that once you capture a city, and it is assimilated, you can produce citizens there that can build and produce things for you. Since you lost so many troops in the attack, you would have had a lot of room under your population cap to build civilians and then build an overseas base.
Putting in a base overseas is still one of the biggest, if not the biggest headaches in actual warfare. I was an army supply officer and had to study how to do it. It is not easy, and does take time, money, material, and manpower. In RoN you have all four to work with.
"Building up the forces for the invasion of Europe took more time and effort than the actual invasion did." Dale Ridder, aka Timerover