The 19th Century and early 20th Century showed numerous examples of the use of military force. Both, “might-makes-right” power plays, and emotional calls to higher ideals.
The military arts were revolutionised during this time period, with new tactics being adopted on the battlefield, and the increasing accuracy of cannon and personal firearms. The explosive power of bombs and howitzers made old fortresses less relevant, and the new fortresses of this period changed their character to be far more sturdy. At sea, everything changed, as the ships themselves increasingly turned into fortresses, with armour plating and technologically advancing naval artillery which was necessary to punch through the copper, iron, or steel plating of the enemy.
A ship which was state-of-the-art in 1836 would become obsolete mere decades later because of changes in armament, protection and, most especially, propulsion. Coal-fired steam engines transformed naval warfare, just as it did world trade and even land-based transportation and factories. Similarly, at the end of Victoria's historical time period, the combustion engine combined with other technologies to give mankind control of the skies.
As never before, wars during this period of history demonstrated the superiority of higher levels of technology against not just primitive forms of technology, but even over weapons and tactics that were standard in the industrialised world mere decades prior.
Victoria 2 allows you to employ your armies as regiments, which can be combined into armies of any size for rampaging across the enemy countryside. They can be loaded onto ships for landings elsewhere, provided they survive the journey against hostile navies of single ships or fleets. Both naval combat and land combat involve strategy and the results will depend upon the technology, strength, leadership, etc. of your military units.