Great Wars are a special type of war in Victoria 2. After the concept of Great Wars has been discovered in the year 1890, any war in which there are at least two great powers on each side will become a Great War. Aside from regular wargoals, Great Wars also have an obligatory war goal: Great War capitulation, meaning that the war leader of the losing side must disarm its military and pay reparations for a certain amount of time, in a manner similar to the "Cut Down to Size" wargoal. Because of this, Great Wars usually require a higher warscore for a negotiated peace compared to normal wars (although, since the Great War Capitulation CB does not have an official cost, enforced peaces at 100% warscore are unaffected). Infamy gained from CBs added in great wars is reduced to one third the normal amount.
Sometimes there can be two great wars simultaneously. In these situations, a third of the great powers might be involved in a Great War and sometimes, all great powers participate in a Great War.
The most common cause for the outbreak of a Great War is a crisis, due to the fact they will often pit multiple great powers against each other. For this reason, a player wishing to prevent the outbreak of a Great War must have mastery of any crises that emerge. Aside from this, however, it can be very difficult to deliberately prevent outbreak. Staying out of a great war, however, is easier, as isolationism can simply be opted for.
Great Wars can have severe effects on a nation's populace and economy, especially when fought on home soil. For this reason it is wise, just like the Great Powers in 1914, to mobilise quickly as soon as war looks inevitable. If possible, a quick drive into the heart of enemy territory once war is declared akin to that of Germany's Schlieffen plan can be a knock-out blow, but it is wise not to repeat the delusions of Germany in 1914; a defensive strategy must be considered.
While it is good to try and create your own strategies to end a Great War it is wise to learn from the actual people who fought in the First World War. Using the landscape around is key - for example, Germany had the very wise decision to take all the high ground for a natural defense in the First World War; this caused massive casualties against impeding British and French attacks. You should always try to create a defensive line on strategically beneficial land. It is also imperative to establish a naval blockade on your enemies' coast for the faster war score increase to end the war sooner.
Since Great powers have colonial nations overseas there is a tendency that they have huge manpower stocks leading to huge stacks of men being hurtled at each other. Getting yourself into a single pitched battle in unfavourable conditions is possibly the biggest mistake ever; it is much better to spread your troops in medium stacks and look to whittle your enemy until a break in their defense appears. This is much better than, say, getting oneself into a "meat-grinder" (that is, a battle involving a huge number of troops with massive casualties on both sides) but, if enough troops are available, it might be a viable strategy to catch the enemy and use it as a distraction. A pitched battle on favourable terrain (such as a mountain) with a skilled general, however, can be much less damaging, and can often be the catalyst for a successful breakthrough.
While this is dark, it must be said that, due to the mass amounts of casualties and damage a great war can do to both the victor and defeated, it is also a valid strategy to undermine your allies. You could not participate in large battles and let allies do most of the heavy lifting. While this will make them the primary victors and also let them determine the goals achieved, to what avail would that be if their manpower stocks were completely wiped? This would leave you, who took a step back, complete dominance of the global stage and the ability to be able to snag those ripe territories of newly discovered rubber and oil. This would leave the resources completely out of the reach of their weakened allies, which in a few years would give you huge amounts of revenue from the automobile and aeroplane industries.
In conclusion, Great Wars can be strategically similar to normal wars. It is worth noting, however, that it is wise to draft war plans in advance and to prepare for a worst-case scenario, as by this point in the game Great Powers are no longer limited to one other Great Power ally each. Whilst certainly possible, in the right situations do not assume that the war will be over by Christmas.
Treaties after Great Wars can be incredibly brutal. It is likely some key territory will be taken from a defeated nation, as well as the obligatory economic reparations demanded. In the position of Great Power, or even war leader, it is easy to take as much as possible from a defeated nation, even to the detriment of your allies who will also have added their own wargoals. It is not wise to be too greedy, as defeat in Great Wars is very humiliating for a nation and will lead to many startling consequences in the future. The same can be said of depriving an ally of a large number of wargoals.
With patch 3.04, a Great War can become a World War after Mass Politics (1905) is researched and the winning side gains 50% or more warscore. After this all wargoals have their cost cut to 30% allowing you to really dismantle stuff at the end of the game.