The bedraggled fighters boarded old, air-conditioned buses with the words “Happy Journey” on the side, and set off through the mountains they had occupied for the past three years.

Usually, they would fight to the death, but this time the ISIS militants negotiated a ceasefire, and a way out.

After a weeklong offensive by the Lebanese army, the Syrian government and Lebanese Hezbollah fighters, over 600 people including ISIS fighters and their families withdrew from their enclave on the Syria-Lebanon border to eastern Syria.

The withdrawal was part of an agreement brokered by Hezbollah, a militant group thought to be more powerful than Lebanon’s own armed forces. The deal included the return of the bodies of eight Lebanese soldiers who were kidnapped three years ago by ISIS.

The border operation brings to an end a three-year ordeal that threatened to drag Lebanon even further into the Syrian conflict.

Lebanon has struggled with a spillover from the conflict next door and has been rocked by persistent bombings tied to the war.

In 2014, ISIS and al-Qaeda militants overran the Lebanese border town of Arsal, kidnapping 30 Lebanese soldiers and sending shockwaves through the country. Sixteen prisoners held by al-Qaeda were released in a prisoner swap in 2015, but five were killed. All nine soldiers held by ISIS were executed.

Hezbollah, a Shiite movement founded in the 1980s to fight the Israeli occupation of Lebanon, joined the war in Syria early on to shore up its ally, President Bashar Assad, and retain a supply line that runs from Iran to Lebanon. Backed by Iran, the group saw Assad’s survival as crucial to its own interests in the region.

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Hezbollah is considered a terrorist organization by the United States and other Western nations, as well as the Arab League.

Critics inside Lebanon say Hezbollah’s involvement in Syria inflamed sectarian tensions at home and brought jihadists to the door.

Hezbollah’s involvement in the operation against ISIS raises some awkward questions for U.S. policymakers.

The U.S. has provided $1.5 billion in security assistance for Lebanon since 2006 to fight ISIS and provide a counterweight to Hezbollah.

Last month, President Trump stood alongside Lebanese Prime Minister Saad Hariri and praised Lebanon for being on the “front lines in the fight against ISIS, al-Qaeda and Hezbollah.”

Trump also pledged more support for Lebanon, saying, “America's assistance can help ensure that the Lebanese army is the only defender Lebanon needs.”

But since then, the Lebanese army has fought in what are essentially joint operations with Hezbollah against ISIS and al-Qaeda.

The Lebanese army has denied acting in coordination with Hezbollah, indirectly or otherwise. This denial comes despite the two forces launching operations simultaneously — the Lebanese military from the east and Hezbollah from the west.

A U.S. State Department spokesperson told PRI that “the Lebanese Armed Forces has been successfully defending Lebanon's borders and fighting ISIS and other extremists on the front lines.”

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On Tuesday, the ISIS fighters arrived in Deir al-Zour province in eastern Syria, near the border with Iraq.

The transfer has angered some in Iraq, who accused Lebanon, Hezbollah and the Syrian government of threatening Iraq’s security by moving ISIS close to their border.

"Any deals or understandings between the warring parties inside Syria or in the region must take into consideration the security of Iraq and not to lead to anything that poses any threat to our national security," Saad al-Hadithi, a spokesman for the Iraqi government, told The Associated Press.

Lebanese Maj. Gen. Abbas Ibrahim defended the deal. According to the Lebanese newspaper, The Daily Star, he told a local radio station that ISIS militants returning “in air-conditioned cars to their countries is permissible because Lebanon adheres to the philosophy of a state that does not exact revenge.”