Forming the HRE
The unification of the Holy Roman Empire (HRE) is the result of the Imperial Reforms series of decisions, added in the third expansion pack, Heir to the Throne. If successful, it unites the HRE members in a single nation, with the Emperor at its head.
Once you form the HRE with the final decision (Renovatio Imperii), you lose all the bonuses from previously enacted reforms, but gain the bonus 'Rome Reborn.'
The requirement for the reforms is Authority, a new concept in Heir to the Throne. This is a measure of the power of the Emperor over the member states of the HRE. There are several ways to acquire Authority. These include:
- Answering the calls of the member states when they are attacked
- Releasing vassals that are part of the Empire
- Getting more provinces to join the Empire (including those of the player)
- Forcing non-member states to relinquish their sphere on member states
- Keeping religious uniformity
- Attacking larger members of the empire and liberating the smaller nations they've annexed (Help Burgundy take over most of France, then turn on them after they add all their provinces to the Empire. The new additions will count as imperial liberations.). If you are aiming for a quick HRE unification, it is advisable not to annex or vassalize other imperial provinces, you'll get them at the unification anyway, and without the prestige hit.
Passing Imperial Reforms revolves around how close you are to the other member states of the empire diplomatically. If you have bad relations or a different religion, it is unlikely that they will endorse any reforms you try to pass. Early on it is best to enter a royal marriage with as many nations as possible, especially the larger ones who are usually more resistant to reforms. Forming an alliance with any electors in geographic proximity to your country is also a good idea until you have a few electors as vassals. The best thing to do is to vassalize the electors early on when they declare war on you through an alliance, as this gives you an extra 250 points towards your HRE rating and insulates any hits you may take from gaining infamy, or losing prestige and legitimacy.
Once you have four or five of the electors vassalized, you are basically guaranteed to win every election, unless you happen to do something incredibly stupid. Because the HRE will throw infamy in your face every year for every HRE non-core province, you should instead rely on vassals and personal unions inside of the HRE, and only expand when through inheritance or a [re]conquest causus belli that gives you cores. Infamy is a big deal when being the emperor and it is best avoided. Having large amounts of vassals and partners in personal unions is also a good idea due to the large amount of alliances you will have. If you are Burgundy, they will fly to your aid against France, if you are Austria, they will save you from Hungary and Bohemia, and if you are Bohemia they will save you from Poland and Lithuania. In this way, you can focus on wiping out the enemy's forces with your armies and using the vassals to pull off the sieges.
Always save before trying to pass HRE reforms. If they don't pass, look at the event window in the bottom left corner of the screen before reloading your game. It will tell you who voted for or against the reform. Make a mental note of those who didn't (probably different culture groups, religions, or those who are not in a royal marriage with you, or those who have a relation of less than 100 with you). Focus on wining and dining those countries. Keep in mind that answering the call to defend HRE and releasing new nations into the HRE will improve your standing with HRE nations.
In the beginning, it is best to gain Imperial Authority by doing things like forcing other nations to release HRE nations, annexing back territories through an imperial ban causus belli, and enforcing religious unity. Towards the end (last four or five reforms) release as much of your nation as a vassal as you can to just breeze through to form the HRE. It doesn't matter how much of your country you whittle away doing this, just make sure the new vassals are on your territories incorporated into the HRE (even if they weren't before, it'll count as a restored HRE nation). You'll get it all back anyway.
The Golden Horde, just like any other Steppe Horde nations are going to declare war upon you whenever the truce is over. Winning a war against them rewards 10 authority, just like winning a defensive war against any out of HRE nations. However, winning the war is easy after the first few decades. If you are the emperor, you should be far ahead in land technology, as nomads usually get struck at technology levels 4-5 for centuries. Also if you are the emperor, you should have enough manpower to make the Golden Horde provide no risk.
Doing this requires some planning. If you shatter the Golden Horde in the first war, they are unlikely to be able hold their territories in Europe, and Lithuania, Poland, the Ottoman Turks, or the Russian principalities will block your access to them. Therefore it is really important not to take too much, or any territories from them, and to accept the first peace they offer. The Eastern European provinces are generally not worth the as much as a united HRE.
This strategy is especially easy, if you are Hungary, Poland or Lithuania, who joined the HRE, or if you are one of the eastern rulers of the HRE (Bohemia, Austria, Brandenburg) and managed to conquer one of the states mentioned before. The best solution is to have personal unions with the Eastern European countries. In this case the nomads need to declare war upon your lesser partner. However this would cause you to be in war with them, even if you don't have a common border. At the moment game mechanics do not allow this, because nomads can only be in war with their neighbours. This helps the nomads to fight their wars against others in Eastern Europe, therefore it prolongs their fall. By the time you integrate the subject nation, you are technologically far ahead, and your manpower large enough to defeat them easily.