Here's what you need to know to get caught up on this story, which could be big news for some time to come:
Now, the hardline kingdom executing people is hardly headline news. But Nimr's death was different.
He was an outspoken advocate for Shiite Muslims in Saudi Arabia's Eastern Province, where Shiites have long complained of discrimination at the hands of the country's Sunni-led monarchy.
Saudi Arabia has long suspected Iran of fomenting dissent among those Shiites, and Nimr's arrest, detention and trial played right into the longstanding animosity between these two regional powers.
What's the deal between these two countries anyway?
It's rooted in the deep divisions among the Sunni and Shiite branches of Islam, but has as much to do with political and economic clout as anything.
The schism dates back 14 centuries, and has to do with disputes over who should succeed the Islamic Prophet Mohammed as leader of the Islamic faith.
Sunni Islam has gone on to dominate the faith -- nearly 90% of the world's Muslims are Sunnis -- including the Wahhabi branch practiced in Saudi Arabia.
In 1979, however, Shiite Islam began to flex its muscles. Revolutionaries in Iran replaced the country's secular government with a theocracy and began to dispatch support to Shiite causes in Lebanon, Iraq and elsewhere across the Middle East.
"The transformation of Iran into an overtly Shia power after the Islamic revolution induced Saudi Arabia to accelerate the propagation of Wahhabism, as both countries revived a centuries-old sectarian rivalry over the true interpretation of Islam," the Council on Foreign Relations writes in an extensive review of the sectarian feud
What are some examples?
For starters, Saudi Arabia backed Iraq's then-Sunni-led government in its bloody eight-year war with Iran.
As that war dragged on, in 1987, Saudi riot police clashed with Iranian pilgrims in Mecca. Some 400 people died -- most of them Iranian Shiites, according to the U.S. Institute of Peace.
Iranian demonstrators attacked Saudi and Kuwaiti embassies. A Saudi diplomat died.
Iran's Supreme Leader, Ayatollah Ruhollah Khomeini, declared the Saudi monarchy "a band of heretics," and with that, the diplomatic lights went off for four years.
The 1997 election of a new Iranian President resulted in warmer ties, culminating in a 2001 security agreement between the two countries.
But things fell apart again in 2003 when the U.S.-led invasion of Iraq toppled Saddam Hussein, unleashed long-suppressed Shiite political power in Iraq and led to closer ties and greater Iranian influence over its neighbor.
Worry over Iran's newfound influence reached fever pitch with the Arab Spring protest movement that began in Tunisia in late 2010. Unrest even reached Saudi Arabia and its ally Bahrain, where a Sunni monarch rules over a predominantly Shiite nation.
Naturally, the Saudis and their allies eyed Iran as a hidden hand behind the upheaval and helped Bahraini authorities quash the uprising.
Last year, relations grew yet colder with conflicts over allegations of Saudi guards assaulting Iranian pilgrims in April and the September deaths of hundreds -- including Iranians -- in twin disasters linked to the annual Islamic pilgrimage to Mecca.
How does all this antagonism play out in the Mideast and around the globe?
In short, it's become a matrix of proxy wars.
In Syria, Iran has thrown in with Bashar al-Assad's government against the Sunni majority -- and Saudi-backed -- rebellion.
In Yemen, where Houthi rebels seeking to overthrow the government are said to have Iran ties, Saudi Arabia organized a coalition of Arab states to bomb rebel targets.
The regional struggle is also being felt in places such as Shiite-majority Lebanon, where Saudi Arabia would like to tamp down the influence of Iranian-backed Hezbollah, and Iraq, where once politically dominant Sunnis now feel disenfranchised.
Will things such as the Iranian nuclear deal and oil prices affect what's happening?
In a place as volatile as the Middle East, there's certainly plenty of incentive to avoid yet another hot war in a region already rife with bloodshed.
It's not out of the question things could get out of hand
, said retired Lt. Gen. Mark Hertling, a CNN military analyst.
"This is spiraling very quickly," he said.
But it's not inevitable.
For starters, it's unlikely Tehran really wants to go to war with well-equipped and U.S.-backed Saudi Arabia and, possibly, its allies.
Iran's military has been crippled by years of economic sanctions and arms embargoes, the Rand Corp.'s Alireza Nader told a congressional subcommittee last month
And even with relaxed sanctions resulting from last year's nuclear deal with Western powers, it's unlikely Iran's economy will grow enough to give Tehran a significant power boost in the region, he said. Conflict with Saudi Arabia wouldn't help that situation any.
Saudi Arabia has internal issues of its own that could limit thirst for aggressive actions.
For one, the country is increasingly feeling the pinch of low oil prices.
And it also appears there's a battle over who will succeed King Salman, who despite ascending to the monarchy last year, may not be ruler for long.
In September, The Guardian
newspaper carried an unsigned open letter from a senior Saudi prince describing a power struggle inside the kingdom and worries that the "the king is not in a stable condition."
What's the rest of the world saying?
In a couple of words, cool it.
China called for "dialogue and negotiations." Russia suggested the nations "show restraint." France asked that the powers "do everything in their power to prevent the exacerbation of sectarian and religious tensions."
Pakistan, a majority Sunni country, condemned the weekend attack on the Saudi embassy in Tehran and urged a "resolution of differences through peaceful means in the larger interest of Muslim unity."
U.N. Secretary-General Ban Ki-moon talked to foreign ministers for both nations Sunday and called the breakdown in relations "deeply worrying."
"What we want to see is the tensions reduced. We want to see dialogue restored, and we want to see diplomatic engagement restored peacefully and without violence," U.S. State Department spokesman John Kirby said Monday.