Peaceful Annexation

In many Total War games, the GM will allow PC nations to peacefully annex other nations into their own. This is generally a fairly difficult thing to do, as few nations will easily give up their independence, however it can be exceedingly rewarding if accomplished. All of the following are variants on the same theme, and the approaches can be mixed provided you can justify the use of the stats to the GM.

Annexation through Diplomacy
In a diplomatic annexation, a nation approaches another nation and asks for a merger between the two. If the other nation accepts, then they join with the PC nation. Although this sounds simple, it usually involves large stat investments for many turns beforehand. A high WO and Morale are necessary for this to have any realistic chance of succeeding, as the other nation will have to believe they will be better off under your rule. Economic investments are also likely to help this succeed. The most important factor however is that the already existing diplomatic relationship between your nations be positive. An enemy will not go for this.

It is possible to use a VIP to have larger chance of succeeding. The VIP would act as a diplomat (Probably having an In-Character talk with the NPC's in question), and he would have to convince the NPC's of following him.

Annexation through Intimidation
Annexation through intimidation is exactly what it sounds. Basically you send an ultimatum to the nation in question, and say that either they will capitulate and submit to your rule, or they will be {insert threat here}. Military - and Reputation if that variation is being used - are the most vital stats. A low Morale on the part of the enemy will also greatly increase the chances of this working.

Annexation through Espionage
In an annexation through Espionage, the annexing nation slowly uses their spies and assassins to take control of the country. This control will for the most part be behind the scenes in the build up to the take over. As the influence in the target nation grows, the annexing nation is more in control, while publicly they appear to have no involvement. Then when the annexing nation thinks that they control enough of the government, they can attempt to seize control of the government. If this succeeds, the nation becomes under their control, though not necessarily in name. They can then initiate a diplomatic annexation at their leisure to both make public and legitimize their takeover, should they feel like doing so.