Warplanes flying out of Saudi Arabia began conducting airstrikes in Yemen on Thursday, and the Egyptian army said it was prepared to join the Saudis in launching a ground invasion of the country.
Yemen has been a mess for years. But how did its domestic chaos — ranging from civil war to a struggle between al-Qaeda and ISIS — turn into a regional nightmare?
Following the end of the Cold War, the formerly divided North and South Yemen unified in 1990, forming the Republic of Yemen. But the president and vice-president — from the north and south respectively — were unwilling to let past grievances go.
The result was a brief but bloody civil war in 1994. President Ali Abdullah Saleh emerged victorious. He spent the rest of the decade solidifying his grasp on power.
After the mujahideen's fight against the Soviets in Afghanistan ended, Yemen was where Osama bin Laden chose to foster the growth of al-Qaeda. From there, the group launched one of its first major attacks against the U.S., targeting the docked USS Cole.
At the start of the "war on terror" in 2001, Saleh decided to ally himself with the U.S., directing it against his political enemies and al-Qaeda alike.
Saleh was both ruthless and extremely savvy, staying in power by balancing his enemies against each other, with his support waxing and waning depending on who was most likely to help or challenge him.
One of the groups that Saleh used, then spurned, comprised the followers of a man named Husayn al-Huthi.
In 2004, al-Huthi — a descendent of what amounts to Yemeni religious royalty, whose followers were part of a Shiite sect of Islam that is close to most Sunni sects — began speaking out in mosques and on the streets after being betrayed. His followers went further, outright denouncing Saleh.
Saleh sent in the Yemeni armed forces to rein in the Huthis. But neither several small wars, the death of al-Huthi, nor a Saudi intervention in 2009 ended the struggle against them.
As the Arab Spring reached its peak in 2011, protests pushed out Saleh, who negotiated a peaceful exit from power. In his place came Abdu Rabu Mansur Hadi.
Hadi took control with international support. U.S. counterterrorism efforts continued. For years, the U.S. has targeted members of al-Qaeda in the Arabian Peninsula (AQAP) — long considered the strongest branch — with armed drones and missile strikes.
Late last year, the Huthis began pushing south from their base in the north towards Sanaa, Yemen’s capital. Last month, they seized control of the city. Last week, they began to push south further still.
Former President Saleh has been aiding the Huthis in their latest push.
The Huthi push towards southern Yemen is also kindling memories of the dormant independence movement in the south. Many in the south have bristled at the reunification ever since the ink was signed on the agreement merging the two.
Amid all this, the region's Sunni powerhouses began worrying that the Huthis' ties to Shiite Islam meant that they were a proxy of their greatest opponent in the region: Iran.
That fear — and the Huthi offensive causing Hadi to flee port city of Aden — led Saudi Arabia to declare on Wednesday night that it would be taking military action to roll back the Huthis.
Saudi Arabia also announced that it had assembled a loose coalition of Sunni states to aid it. While the U.S. isn't directly getting militarily involved, it is backing the Saudi initiative and coordinating closely with it.
While most of the support is coming from Gulf countries, countries further from Yemen such as Egypt, Pakistan, and Sudan are also pledging support.
As if the regional dynamic wasn't enough, there's a battle brewing between AQAP and ISIS for control of the jihadi movement in Yemen. Last week, a Yemeni branch of ISIS claimed credit for a mosque bombing that killed at least 126 people.
For now the Saudi-led offensive is limited to airstrikes. But Egypt and Saudi Arabia said they are considering launching a ground invasion as well, though how this will restore Hadi to power remains unclear.
As the strikes continue, this all leaves several questions up in the air: Will the campaign draw the Huthis to the bargaining table? How will the Yemeni people respond? And will ISIS and AQAP take advantage of the ever-escalating chaos?
The attacks on the U.S. Embassies in Kenya and Tanzania in 1998 were the first major al-Qaeda attacks on the United States. A previous version of this story misstated that the attack on the USS Cole was the first.