It's just hard to believe that the dispute is only about a 14 hundred year-old argument about who should be in charge.

Well, yes and no. If you boil down the argument to a simplistic level, then yes, it is about an argument over who should be in charge after Muhammad. Which seems innocuous at first glance.

On the other hand, that above interpretation ignores the ramifications of said rift. The main one being the Hadith, or the reports by others regarding the words of the Prophet. The Hadith is important as it also serves as a means of clarification regarding Islamic jurisprudence and tenets.

The Hadith is not only a collection of narratives regarding the Prophet written by others, but also details the chain from where the narratives originated. Now, here is where the debate regarding succession plays a critical part in the Sunni vs Shia issue - both branches follow very different Hadiths, because some narrators are seen by the other side as untrustworthy. For instance, the Sunni accept the words of Abu Bakr and his supporters; the Shia instead prefer the words of Ali and his supporters.

What does this all mean in practical terms? It means both branches have very different methods of worship and interpretations of Islamic jurisprudence. Two examples would be in how they pray, and how marriages are arranged [i.e. Shia allow fixed-term marriages, Sunni completely reject this concept].

So on that point alone, regarding what is "proper" Islamic practice...that has been the source of much conflict. However, there have been points where Sunni and Shia have cooperated as well, so hasn't been all pain and bloodshed.

ETA: The Battle of Karbala was the first major battle between Sunni and Shia and caused a lot of division and animosity between the two groups. Apologies for the Wiki link but it seemed to be a good general summary of the battle itself.

I'm sure there are scholars of Islamic History who can explain the earlier events better than I can, but what I find significant is that the Shia-Sunni divide has not always been as divisive as it is today. Not that long ago intermarriage between Sunnis and Shias was common across the Middle East. The big event in contemporary history seems to be the Iranian Revolution in 1978/79.

Khomeini's revolution was not initially a solely Shia revolution, but an Islamic revolution. Faced with a radical challenge to both their religious and political legitimacy, Gulf Monarchies began to promote the Shia-Sunni divide as a means of undermining Khomeini's appeal. For example, they funded the writing of anti-Shia polemics and accused any opposition of being part of a 'Shia conspiracy'. The Iranian government soon joined in on the promotion of sectarian rhetoric. Vali Nasr's book 'The Shia Revival' goes into a lot of detail on this point.

hi! you'll find more info in these posts

• What are the roots of the divide between Sunni Muslims and Shiite Muslims?

• Why exactly do the Sunni's and the Shiites hate each other so much?

• What precipitated the split of Sunni and Shia Islam?

• How and why did the Sunni/Shi'a schism become a highly divisive issue in the Arab world?

• When, why and how did the Sunni/Shi'a split become a theological, and not just political, divide?

Since the split between Sunnis and Shia there have been long periods of brutal conflict between the two sides and long periods of detente.

In the early 20th century when modern Islamism first began developing it largely took a pan-Islamic/non-sectarian approach. Early Muslim Brotherhood figures (who are Sunnis) considered Shia Muslims and the Dawa Party (a Shia Islamist group which now runs Iraq) initially had Sunni founders. During the early/mid 20th century Islamists spent most of their time confronting Communists and secular Pan-Arabists who were the dominant political forces in the region at the time.

When Khomeini (who was a Shia cleric) took power in Iran he furthered this Pan-Islamic world view. He was very interested in spreading his revolution across the Islamic world and didn't want to play up the sectarian divide. He supported Sunni Palestinian groups for example. He also decried almost all existing Arab regimes and offered assistance to Islamist groups fighting them.

Around this time (late '70s/'80s) the Saudi Arabian government was starting to go from rich to ridiculously, obscenely rich. The Saudis are Salafiyn/Wahabbiyn and so have never liked Shia, but the fact that Shia leaders in Iran were now calling out the Saudi royal family for it's corruption and hypocrisy and generally making them look bad put the Saudi propaganda machine into overdrive.

The Saudis were funding Salafi groups across the Muslim world on a massive scale, and during the '80s and '90s these groups began to redouble their anti-Shia rhetoric. This anti-Shia stance was strongly supported by the USA and "secular" Arab regimes who were threatened by Iran and it's revolutionary/destabilizing foreign policy. This anti-Shia propaganda was successful and these days it's not uncommon for normal, non-Salafi Sunni Arabs to causally express the belief that Shia are not Muslims.

This growing beef has been exacerbated by the fact that Sunni rulers in countries like Lebanon, Iraq and Saudi Arabia have historically disenfranchised Shia populations in their countries. Iran took advantage of this and funded Shia groups/militias to challenge this discrimination. Saudis and Gulfis have responded to this by funding various Sunni groups/militias to fight the Shia. When these groups inevitably slaughter members of the other sect it further fans the flames of sectarian hatred.