Answer 1
They are not fighting. The basic beliefs of both are the same. They follow the same Prophet (SAW). Only the extremists sometimes somewhere create problems.
Answer 2
It depends on when the conflict happened and entirely on which Shiites and which Sunnis are fighting.
Many of the historic Sunni Muslim Empires actively repressed and persecuted Shiite Muslims. The seminal event beginning this persecution was when Yazid I of the Umayyad Caliphate ordered the execution of Imam Hussein and a large remnant of the Ahl al-Bayt (Prophet Mohammed's family). At this point in time, the Shiites were a political faction supporting Ahl al-Bayt over the Umayyad Caliphs. The murder of Hussein began to transform the identity of the Shiites and plays a central role in their beliefs. The Umayyad Army followed through with this. Additionally the Umayyad government wanted to create a Sunni Arab aristocracy and therefore applied the jiyza tax not only to Jews and Christians, but to Mawali (non-Arab Muslims) and the Shiite Muslims, regardless of whether they were Arab or Mawali. Shiite Muslims were also banned from most government positions under the Umayyads. Finally, the Umayyads continued to seek out the Shiite Infallible Imams, torture them and murder them.
Shiite Muslims joined arms with the Sunni Abbassids in 750 CE who promised a better situation for the Shiite Muslims, but they were later deceived. The Abbassid Caliphs continued the Umayyad trend of torturing and murdering the Shiite Infallible Imams, but extended this as well to Shiite leadership (Mullahs and Faqihs). Shiite Mosques were destroyed and worshipers during Ashura processions were murdered. Shiite Muslims and their property were also periodically attacked as scapegoats for Abbassid military problems, such as the Byzantine offensive in 971 CE. The Abbassid Caliphs also provided financial support to those Faqihs (Islamic Jurists), especially from the Hanbali School, that would propagate anti-Shiite attitudes.
The Ottoman Empire saw the Shiites under its purview incorrectly as a fifth column for its rival in Persia, the Safavid Empire, which was a Shiite Islamic State. To prevent Shiite Muslims from becoming a critical mass in the country, the Ottomans massacred large numbers of Shiites, especially the Turkish Alevis, the Syrian Alawites, and many Lebanese Shiite Muslims (mostly Twelvers).
However, the Shiites have also persecuted Sunnis under their watch. The first major attack of Shiites against Sunni leadership was in the final days of the Fatimid Caliphate when Fatimid rulers created the Assassins who took down many notable Sunni leaders such as Grand Vizier Nizam ul-Mulk of Baghdad, Mohammed Ghori, the Atabegs Maudud and Zengi of Mosul. The Assassins even targeted Saladin the Ayyubite, but were unsuccessful.
The Persian Safavids fought several wars against neighboring Sunnis in Samarqand to the North and the Ottomans to the West. Ismail I (the first Safavid) adopted Twelver Shiite Islam and began to persecute the Sunnis in Iran. This reduced their community to a small minority in the Persian heartland. He destroyed numerous Sunni mosques and grave sites as well as mandating curses against the first three Rightly-Guided Caliphs. He also imprisoned and killed large populations of Sunnis for their beliefs and compelled conversion to Shiite Islam through violence. The Safavids also spread this form of Sunni oppression through conquest Azerbaijan and of southern Iraq and imposing conversion to Shiite Islam there as well.
Answer 3
Not all Sunnis and Shias hate each other, nor do they fight everywhere.
The Sunnis have no objection to the beliefs of Shias like:
Imamat or the way they offer prayer or the Kalma they recite, or Muttaa (temporary marriage), taqqiya, so and so on. The thing that provokes Sunnis to fight against Shias is:
The derogatory, insulting and offensive remarks and filthy uttering of Shias against the holy Companions (RAU) whom Almighty Allah has many times mentioned as the criterion of Eeman (Faith). In Sura Fateh 1400 Companions (RAU) woved to sacrifice their lives at the hands of the Holy Prophet (SAW) to avenge the rumored about death of Hazrat Usman (RAU). Almighty Allah likes that wove and appreciates the Companions (RAU). The holy Companions of the Prophet (SAW) sacrificed their lives, properties and even kith and kin for the sake of Islam. Almighty Allah has given them a certificate of His pleasure in the following words:
Translation: Allah is pleased with them and they are pleased with Him.
The Shias curse them, and also curse the wives (Mothers of all Muslims) of the holy Prophet (SAW). How can a true Muslim tolerate his Mothers being cursed and blamed, and their effigies being burned and thrown shoes at? How can a true Muslim tolerate Hazrat Abu Bakr RAU and Hazrat Omar RAU being disgraced publically and cursed on loudspeakers? It was Hazrat Abu Bakr RAU whom the holy Prophet (SW) appointed to lead the Muslims in their prayers. It was not Hazrat Ali RAU or even Hazrat Abbas RAU, the beloved Uncle of Hazrat Muhammad (SAW). Worldly Caliphate is not inherited by Prophets. The Companions RAU chose Hazrat Abu Bakr RAU their Caliph who proved the only suitable choice for this important duty.
This is the only cause of fighting between the two groups. Shiaism is the only religion in which abusing and cursing the holy personalities of Islam is the basic belief. Keeping one's true ideas and faith hidden (Taqiyya) is also considered to be their basic belief. Hazrat Ali RAU, Hazrat Hassan RAU, Hazrat Hussain RAU never started doing such nefarious and immoral activities, neither did they initiated Ashoora processions, beating their chests and wounding them with chains. Islam does not permit hateful remarks and such things even against Hinduism, Budhism or any other religion. The holy Prophet (SAW) is Rahmat (Mercy) for all creatures of all Ages. He never taught or preached cursing others. Those who claim to be Muslims should surrender unconditionally to the Will of Almighty Allah, Who has created us all and loves us all, humans, birds, animals, fish and even plants. Let us tolerate humans belonging to different religions. Each of us is answerable to Allah Almighty for his own deeds, not of others.the Sunnis are the majority and the shiites are the minority.therefore the numbers of Sunnis are more than shiites populations.
Sunnis, Shiites and Kurds
No, Shiites outnumber Sunnis in Iraq. However, Sunnis over the world outnumber Shiites. World statistics is that Sunnis Muslim world percentage is 85% of total world Muslims. While Shiites are less than 15%. Refer to question below for more information.
No They are not Sunnis. They are Shiites.
The Sunnis are of bigger population. Of world Muslims, 85% are Sunnis.
661AD
Shiites and Sunnis. However, they are not splits as for example in Chritianity different denominations. Sunnis and Shiites differe in minor Islam details. Refer to question below.
Neither of them is fighting. The basic beliefs of both are the same. They follow the same Prophet (SAW). Only the extremists sometimes somewhere create problems.
Sunnis and Shiites
Both Shiites and Sunnis are right Muslims. They differ in minor issues that are not relevant to the basic Islam rules and instructions. Refer to related question below.
Shiites and Sunni are two main schools in Islam religion. Shiites are Muslims as Sunnis are Muslims too.Shiites and Sunnis are both Muslims and have different views in some minor and side issues.They are not different as the Catholics and orthodox for example.Both Shiites and Sunnis recite the same Qur'an, believe in same prophet, practice same ritual worships, and go to same places for hajj (or pilgrimage).Refer to related question below.
Shiites and Sunnis