It is a powerful automatic casus belli that does not need to be justified.
Prerequisites
The Assert Hegemony casus belli is only available under very specific circumstances:
You must be a Great Power. Another Great Power must exist within the same culture group. Said Great Power must have a nation of the same culture group in its sphere of influence. Said Great Power must not be a cultural union, such as Italy or Germany. You must have researched the Nationalism and Imperialism technology.
The first one is trivial: you will eventually be a Great Power in all of your games. The second, however, is already a problem: how often do two great powers emerge in the same culture group? Let’s look at the Great Powers:
France and Great Britain have no competitors: no countries of similar culture will emerge and rise to the status of Great Power in a standard game. The same goes for Russia, the USA, and the Ottomans. Spain has a potential rival in Portugal, but there is no third country of Iberian culture to be sphered by either. Non-westernized nations such as China, Japan, and Persia will usually conquer nations in the same culture group. No eligible Great Power will remain.
All the countries above are highly unlikely to be able to use Assert Hegemony during a standard playthrough. All the conditions can technically be verified in Italy. Should Sardinia-Piedmont and the Kingdom of Two Sicilies become Great Powers pre-unification and acquire spherelings, such as Lucca and Tuscany, they could use the casus belli. This is improbable as well. The only remaining candidates are the Germanic Great Powers: Prussia and Austria. They both start with spherelings of their culture group, with Prussia controlling most of the North German states and Austria controlling the South German ones and Saxony.
Nationalism and Imperialism
The National and Imperialism technology can only be researched after 1850: it is not available right from the start of the grand campaign. It is an important technology, providing an additional national focus to a sufficiently populous nation and boosting several casus belli, such as Establish Protectorate. It should be high on your priority list, and if you are planning to use the Assert Hegemony casus belli it is indispensable.
Declaring War
The casus belli itself does not need to be justified. If enforced, it will remove all nations within the shared culture group from the defender’s sphere of influence. It is a comparatively powerful effect because there is no limit to the number of satellites affected. However, said satellites will not be added to your sphere of influence and you will not be able to add them for at least five years, the length of the truce.
Is Assert Hegemony Worth It?
It depends. Yes, this is a terrible answer to the question, so let’s study the situation on a case by case basis:
As Italy, you should have unified the peninsula ten years before Assert Hegemony becomes available, so it is never worth it. If you got terrible luck and you have the option to use this casus belli, you might as well reroll the run. As Prussia (or the Northern German Federation), Assert Hegemony is not always a good choice. Austria only has three satellites you care about, and forcefully adding them to your sphere costs less than fifteen infamy points. You should evaluate whether you can fight a prolonged war with Austria. As Austria, the casus belli is very powerful, provided that you managed to keep Saxony in your sphere of influence. Prussia has a lot more than three satellites in Germany. If you take them away, you will also isolate Prussia proper from the Rhineland, greatly weakening your main rival.
In conclusion, Assert Hegemony is a powerful, but very situational casus belli. In its niche, it can be game-changing, and now you know how it works.
