Shiasm

________SHIA EPISTEMOLOGY___________________________
While deep Sufism sometimes regards chains in Hadiths as exoteric and unnecessary (the Hidden Treasure Hadith for example is considered impeccable by Sufis though it has no chain at all). Shia however disregard what they consider strong and even true Hadiths while also holding high regard for Hadiths they sometimes regard to by profligate liars. The reasoning here is due to a unique epistemology best explained in their own words, which are quoted from their scholars, links provided

"We shall learn in the coming lessons that sometimes a Hadith is weak as far the chain of the narrators are concerned, yet the scholars accept its content. On the other hand, there may be an authentic Hadith as far as the chain of its narrators is concerned. Yet the scholars do not rely on it. For if a narrator is weak it does not mean he would never tell the truth. Similarly, it could be that a Hadith is truly narrated from one of the Imams (a.s.), yet the Imam did not have a real intention in expressing it such as the Ahadith of Taqiyyah."

Thus for example very strong Hadiths which show Ali, رضي الله عنه, or Ja'far praising Abu Bakr and Umar, رضي الله عنما, are generally disregarded as Taqiyyah on his part (including Ali marrying his daughter to Umar which they think was to trick Umar), whereas Hadiths supporting Shia doctrine even if narrated by those Shia regard as liars are considered true. The concept of Taqiyyah thus is not just a practice but extremely important to Shia epistemology. This is the basis for narrations about Umar brutalizing Fatima, رضي الله عنها, and that sort of thing, which Sunnis reject.

"Al-Taqiyyah (protection) is a Quranic principle that the Shia believe in. There are quite a number of Ahadith that are expressed by the Imams of Ahlul-Bayt (a.s.) with the intention of protecting the Shia. Although these Ahadith could have been the actual words of the Imams (a.s.) the Shia scholars do not act upon them for they Imams (a.s.) had expressed it for certain circumstances only."
https://www.al-islam.org/sciences-hadith-shaykh-mansour-leghaei/meeting-5-causes-weak-ahadith-books-shia

This is actually a very good series by a Shia scholar explaining Shia epistemology which is really at the root of the Shia Sunni split
https://www.al-islam.org/sciences-hadith-shaykh-mansour-leghaei/meeting-5-causes-weak-ahadith-books-shia

_______SHIA POLITICAL THOUGHT_______________________
Within Shia political theory, the high jurist acts on behalf of the hidden imam, who is believed to have total and absolute authority over all creation, and thus the high jurist can actually overrule the Qur'an (which in Shia belief is created)

"Is he only able to issue orders within the framework of the Islamic legal system, or is he fully authorized to make decisions even if they contradict the Shari’ah? In other words, is his license as a ruler defined by the Shari’ah, or is his authority above the Shari’ah and therefore absolute?"

[....]

[Khomeini] writes:

"The government or the absolute guardianship (al- Wilayat al-mutlaqa) that is delegated to the noblest messenger of Allah is the most important divine laws and has priority over all other ordinances of the law. If the powers of the government restricted to the framework of ordinances of the law then the delegation of the authority to the Prophet would be a senseless phenomenon. I have to say that government is a branch of the Prophet's absolute Wilayat and one of the primary (first order) rules of Islam that has priority over all ordinances of the law even praying, fasting and Hajj...The Islamic State could prevent implementation of everything - devotional and non- devotional - that so long as it seems against Islam's interests."

This is a serious political conflict with Sunni thought though because within that framework Islam is submission to Allah so you could not coherently defy Allah in the interests of submitting to Him.

"Unlike conditional authority (Wilayat al-muqayada) that restricts the right of the faqih for issuing governmental orders solely in permissibility cases (mubahat), Wilayat al- mutlaqa, by definition, is a juridical view concerning the dominion of the just faqih to issue governmental orders even if it is in opposition with some obligatory Islamic laws."

Full article, worth the read
https://www.al-islam.org/shia-political-thought-ahmed-vaezi/what-wilayat-al-faqih

__________SUNNI AND SHIA DIFFERENCES_______________
Shia regard almost all of the companions of Muhammad ﷺ as false Muslims, including A'isha, 'Umar, Abu Bakr', رضي الله عنهم, and the overwhelming majority. This means that most Sunni Hadiths, being reported by (in Shia eyes) fake Muslims, are considered unreliable by Shia, who rely on a different canon of Hadiths which Sunnis themselves consider unreliable. Further complicating the matter is when it comes to the few companions Sunnis and Shia both hold in high regard, there is a very different interpretation of what's mutually agreed upon. For example when Ali, رضي الله عنه, publicly praised Umar and Abu Bakr, Sunnis take it at face value, whereas according to many Shia sources this was taqiyyah on his part, and secretly he said they are cursed, whereas Sunnis reject these secret narrations; to Shia this is similar to the Qur'an having an exoteric and esoteric element, Ali in an exoteric statement praised the Sahaba but for those initiated into the esoteric truth he hated said companions. Lately another school of Shia thought has emerged which rejects the narrative against most of the companions of the Prophet ﷺ, and sincerely desires a brotherhood with Sunnis. Khomeini for example forbade cursing Umar saying this is wrong. The difficulty with this approach is that logically it would require one to accept the Hadiths narrated by most companions like A'isha, which would in effect bring Shia jurisprudence and theology in alignment with Sunni (although still a distinct school of thought within Sunnism), and would require rejecting many very important Shia Hadith narrators as fabricators. Therefore while Khomeini's thought is very important to contemporary Shia discourse, many Shia scholars reject his particular stance here as inconsistent with Shia canons of Hadiths. To give an example, pleasure marriage--marriage contracted to expire after a few days or hours--was a common Arab practice, but according to A'isha, Muhammad ﷺ eventually banned it, and Abu Bakr himself holding this view cracked down on it as prostitution. Shia traditionally discounted these opinions because they regard A'isha and Abu Bakr as deceitful; thus pleasure marriage is still permissible by Shia jurisprudence, and a change in opinion on A'isha or Abu Bakr would logically entail a change here. Another example is Khomeini's ruling that sex changes are permissible, this is based on the idea that one can be an exoteric man but an esoteric woman and that while the material world isn't the spiritual (as I have stated before, Shia metaphysics are somewhat akin to Gnostic whereas Sunni metaphysics are more akin to presocratic) it can be made to reflect it. Sunnis however reject this ruling because A'isha reported that Muhammad ﷺ cursed men who dress as women and women who dress as men, and there are several other reports of this but by people Shia traditionally considered fake Muslims. Thus Khomeini's revision that the companions were in fact sincere and reliable Muslims would pose the problem of reconciling his jurisprudence with their reports.

_________POLITICAL TROUBLES____________________
The political problems really kicked off when a Sufi order, the Safavids, embraced Shiaism, and purged the Sunnis of Persia, installing Arab Shia in their place, thus making Persia Shia. The Wahhabis rose some time after this and it was naturally a feud. In terms of today the Shia fighting alongside America in Iraq and Russia in Syria has further caused tension.