The targeted killing of Iranian Gen. Qassem Soleimani brought into focus Iran's secretive Quds Force which he headed.
Iran has vowed to avenge the death of one of its most powerful military and political figures, who was the head of the country's secretive Quds Force.
What is the Quds Force?
The Quds Force is part of Iran's Islamic Revolutionary Guard Corps (IRGC), according to the Council on Foreign Relations (CFR).
Founded after Iran's 1979 revolution, the IRGC defends the Islamic Republic.
The Quds Force performs "operations external to Iran to advance the Islamic revolution," Dr. Jack Watling, a research fellow at British think tank, the Royal United Services Institute (RUSI), told NBC News.
Where are Quds Forces based?
Run from Tehran, the Quds Force has developed ties with armed groups in Afghanistan, Iraq, Lebanon, Syria and the Palestinian territories.
Iranian operatives were blamed for both the 1983 bombing of the U.S. Embassy in Beirut and the attack on barracks housing American and French service members, which claimed the lives of 307 people.
Later, President George W. Bush said Quds Forces coordinated with Shiite militants to plant roadside bombs to kill U.S. troops in Iraq.
More recently, its members have served as military advisers to forces loyal to Syrian President Bashar al-Assad, according to the CFR.
After the Islamic State militant group (ISIS) gained a foothold in both Syria and Iraq, they also helped to mobilize and lead tens-of-thousands of Shiite militiamen against the terror group.
How big is the Quds Force?
Estimates vary, but Watling said it was likely a "divisional strength military formation" of around 17,000 to 21,000 members, "which is split into regionally aligned brigades of soldiers."
But he said the numbers become "very vague" because it can pull in people from the wider IRGC and proxy actors.
"When they put teams together and get technical experts or media experts, those people might not be trained soldiers who are part of that regionally aligned brigade structure," Watling said. "They might be from elsewhere, but they're recruited on a mission basis."
As a result "numbers become quite difficult to predict," he added. "It is just difficult to breakdown who is an employee of the Quds Force rather than who is an agent of the Quds Force."
What impact will Soleimani's death have on the Quds Force?
His death "is unlikely to disrupt Iran's operational capabilities," Dr. Aniseh Bassiri Tabrizi, another research fellow at Royal United Services Institute, told NBC News.
She added that it was "likely to continue carrying out its previous strategy in terms of reliance on proxies and non-conventional capabilities."
Iran acted quickly to replace Soleimani but whether his successor, Gen. Esmail Ghaani, can be as effective is open for debate.
"One of the standard challenges in all these proxy relationships is that the proxy wants to serve their interests rather than the patron's interests," Professor Stephen Biddle, an adjunct senior fellow at the CFR told NBC News.
"One of the things that Soleimani was surely unusually good at was managing those relationships. His personality was surely important in that process. So, with Soleimani gone, a replacement who is unlikely to have his personal charisma probably won't be as effective at the margins."
Who is Gen. Esmail Ghaani?
While Ghaani has comparable military experience to Soleimani, having served extensively in the war between Iran and Iraq during the 1980s, compared to his predecessor, Watling said he "is quite quiet, quite reserved and when he does speak publicly it tends to be from a script."
Ghaani, who served in counterintelligence, had a "very established personal relationship" with Iran's Supreme Leader Ayatollah Ali Khamenei, he said.
"He was one of the critical liaisons between Soleimani and Iran's supreme leadership, so if anything that makes that tie closer," he added.
Whereas Soleimani was very focused on Iraq, Syria and Lebanon, Watling said that Ghaani had worked mostly in Afghanistan and Pakistan.
"The picture is a quite competent but reserve manager… He has quite a different style of leadership" he added.