For several months now, a civil war has been raging in the impoverished nation of Yemen on the Arabian Peninsula. While tensions in the region have simmered for decades, this most recent conflict broke out when a group of Shia rebels known as the Houthis overthrew the Sunni elected president of Yemen Abd Rabbuh Mansur Hadi earlier this year. They also took control of most of Yemen's port city of Aden. Now, nearly seven million of Yemen's people are facing a famine crisis. Though the United Nations recently took advantage of a temporary ceasefire to deliver food and supplies to the civilian population, more suffering will surely follow until this power struggle is resolved. So nearly half a year after the outbreak of this conflict, where do things currently stand?
The Houthis have been driven out of most of their strongholds within Aden and are being pushed back into the countryside of Yemen. With Saudi Arabia leading airstrikes on Houthi safehouses, Sunni tribal leaders engaged in ground combat, and a large Saudi Arabian army amassed at the border with Yemen, it appears the Houthi's days in charge of Aden (and much of its remaining territory) are numbered. It should be noted that the Houthis never controlled Yemen outright even after overthrowing President Hadi's government. Yemen's political makeup has a strong emphasis on regional alliances and local officials, so overthrowing a president doesn't exactly guarantee control over the entire country. Many have compared this current civil war to the much longer divide between Sunni and Shia Muslims throughout the world. This is a vast oversimplification and greatly overemphasizes the role of nations like Saudi Arabia and Iran in determining local politics. Still, the Sunni-Shia divide plays a big part of this conflict. Let's examine the (very oversimplified) basics of this division in Islam and what it means.
Following the death of Islam's prophet Muhammad, a new leader (Caliph) of the Muslim community needed to be appointed. One group, who would later become the Sunni (from the Arabic "Sunna" meaning the sayings and actions of Muhammad) believed that a man named Abu Bakr should lead the community since he was likely the closest companion to Muhammad. However, some people (who would later become know as the Shia meaning followers of Ali) disagreed, believing that only a member of the prophet's family could rightly hold the title. They wanted to see a man named Ali ibn Abi Talib become Caliph instead (Ali was Muhammad's first cousin and married Muhammad's daughter Fatima). Though they got their wish a few decades later when the Caliph Uthman ibn Affan was assassinated, the struggle of trying to make a member of Muhammad's family the leader of the Islamic community caused these factions to drift further apart and ultimately break out in civil war (known as the Fitna).
Since then, many Sunnis have generally recognized (or at least acknowledged) a long series of Caliphs. These were usually the leaders of whichever conquering army was in control of the region at the time (Umayyad rulers, then the Abbasid rulers, then the Ottoman sultans). The position of Caliph disappeared entirely when Mustafa Kemal Ataturk abolished the office in Turkey after declaring Turkish independence from the Ottomans. No widely recognized leader has taken up the position since then (though Abu Bakr al-Baghdadi of ISIS has tried to claim this title).
The Shia, on the other hand, began recognizing a series of Imams (like an Islamic priest) as their spiritual leaders. Most Shia recognize twelve Imams as being divinely appointed (the twelfth Imam supposedly went into hiding centuries ago and will return as a savior of humanity), though some only recognize the first five or seven of these as divinely inspired. This may seem like a minor division between Sunni and Shia, but centuries of violence between some in these factions has caused distrust and fueled revenge attacks in a long cycle of conflict. With nations like Iran being predominantly Shia and the Arabian Gulf nations being predominantly Sunni, ethnicity and nationalism can also play a part in this mistrust. However, we shouldn't imagine that all Sunni and Shia do not like each other. These are only broad trends and the majority of average Muslims are not quite as concerned about these religious differences as media sources would like people to believe. The divide has existed for well over a thousand years, but this does not mean Sunnis and Shias have been embroiled in perpetual conflict for the entire time.
So can the two live peacefully together? Yes, under the right circumstances. In places like Lebanon, the Sunni and Shia populations (along with some Christian minorities) have worked out a power sharing plan within the government to ensure each group has a say in the political process. In places where Muslims are not the majority, Sunni and Shia populations often have to join together to advocate for their common rights and religious freedoms in the face of discrimination. The main contributor to positive Sunni-Shia relations appears to be a more representative system where one faction does not have complete and perpetual dominance over the other. However, with Iran, Saudi Arabia, and ISIS all encouraging Sunni-Shia conflict, it is very unlikely a genuine peace will emerge anytime soon.
What does this divide mean for Yemen's current conflict? Assuming the Houthis are eventually defeated (or at least pushed back into the fringe of Yemeni politics), the country will likely remain a close ally to both the United States and Saudi Arabia. Iran would also see its spreading influence blocked in Yemen (though it is important to recognize again that the Houthis are their own independent actors and not just puppets of the Iranians). Unfortunately, this conflict has allowed AQAP to spread and take advantage of the chaos of the conflict to recruit more members. Though the terrorist group has very limited ability to conduct attacks outside of its small regional area, it will continue to gain strength the longer this conflict drags on.
No matter the outcome of this civil war, Yemen's sectarian problems will never be resolved without the economic, educational, and infrastructural changes which it desperately needs. These reforms alone will not solve all of Yemen's problems, but an educated and productive population is far less likely to become mired in the patters of cyclical violence which often plague poor regions with divided societies. The "great Sunni-Shia divide" is not an impossible gulf to bridge. With time and enough positive investment, even these long-competing groups can learn to coexist (or at least tolerate one another).
TL;DR: The Sunni and Shia have (on a broad scale) opposed each other for many centuries, but that is no reason to abandon all hope of a great Islamic coexistence.