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The U.S. and Iran, rival powers in Middle Eastern affairs, are once again working together to combat the Islamic State militant group (ISIS) via local allies, this time in the barrens of the Lebanese-Syrian border. Their cooperation follows a previous, reluctant understanding in Iraq.

In the latest grouping, the U.S.-backed Lebanese army is set to storm ISIS outposts tucked inthe mountains of Ras Baalbek that separate Lebanon from Syria, where a civil war between the government and rebels has allowed jihadists and other militants to threaten regional security. Iran has been amajor supporter of Syrian President Bashar al-Assad and his armed forces, bolstering their ranks with its Lebanese ally, Shiite Muslim militant movement Hezbollah. While considered a terrorist organization by the U.S., Hezbollah has proven an effective force against fighters with both ISIS and Al-Qaeda, and appears poised to join the fight. As a final showdown looms, Lebanon has prepared for a deadly confrontation.

Related: Trump didn't know what Hezbollah was; militant group responds by saying U.S. stands with ISIS

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"We are approaching a time that will see blood and martyrs," Lebanese Foreign Minister Gebran Bassil told families living in the town of Qaa, where ISIS recently shelled Lebanese army positions, according to The Daily Star.

Lebanese army soldiers take part in a parade at a military academy marking the 72nd Army Day in Fayadyeh, near Beirut, August 1, 2017. The Lebanese army and Shiite Muslim militant group Hezbollah have a complex relationship, but both have sought to secure the country's borders from jihadists attempting to enter from Syria. Aziz Taher/Reuters

The Lebanese army has repeatedly denied that Hezbollah would participate in the operation. The force reportedly received about $80 million in equipment and training from the U.S. last year, boosting the total in the past decade to nearly $1 billion. The U.S. has been extremely critical of Hezbollah's participation in the Lebanese government, where the group forms one of the largest political parties. President Donald Trump referred to Hezbollah as a "menace" during a press conference late last month with Lebanese Prime Minister Saad Hariri, another political opponent of Hezbollah.

Hezbollah, however, has already made a sizable contribution to the upcoming offensive against ISIS. Late last month, the Lebanese fighters teamed up with the Syrian military to launch a dual assault on militants loyal to Hayat Tahrir al-Sham, formerly known as the Nusra Front of Al-Qaeda, holed up near the border village of Arsal. The roughly week-long operation ended with a ceasefire and total expulsion of the jihadists into rebel-held territory in northwestern Syria. Following the victory, Hariri said Hezbollah "has accomplished something and what's important is the result," according to LBC News.

In a televised address to his supporters, Hezbollah Secretary General Hassan Nasrallah said Friday his group would hand overparts of Arsal its fighters were stationed in at the army's request and thanked allies Lebanese President Michel Aoun, Assad and Iran for their support in the battle. Contradicting statements from the Lebanese army and Hariri, Nasrallah said his forces would begin a parallel offensive against ISIS from the Syrian side of the border, using the Arabic-language acronym for the group, Daesh.

"The Lebanese army will be responsible for the operation against Daesh, but the request for American aid is an insult to the Lebanese army," Nasrallah said, according to Lebanon'sEl Nashra.

"We in Hezbollah are at the service of the Lebanese army while on Lebanese territory," he added. "Meanwhile, Hezbollah and the Syrian army will open a Syrian front against Daesh. Its timing is in the hands of the Lebanese army, and we are ready."

A Hezbollah fighter stands in front of anti-tank artillery at the barrens of Arsal, near the Syria-Lebanon border. The Lebanese Shiite Muslim militant group's success has paved the way for an upcoming Lebanese army assault on Islamic State militant group (ISIS) positions nearby Ras Baalbek and Hezbollah said it would play a role from Syria. Ali Hashisho/Reuters

The alignment of U.S. and Iranian tactical interests in Lebanon mimics that in Iraq, where both countries devoted extensive resources to battling ISIS despite significant political differences. After evolving out of Al-Qaeda in Iraq's insurgency against the U.S. military and local Shiite Muslims, ISIS managed to take nearly half the country before spreading into neighboring Syria. The U.S. responded by forming an international coalition to launch airstrikes against the jihadists while at the same time supporting the Iraqi military and Kurdish forces on the ground. Iran stepped in by backing a number of majority-Shiite Muslim militias known collectively as the Popular Mobilization Forces.

As in Lebanon, the sectarian nature of the forces involved in Iraq has sparked concerns as to how the country will be governed post-conflict. The central governmentin Baghdad continues to hold political power, but recognition of the Iran-backed Popular Mobilization Forces as an official military wing of the state and Kurdish calls for independence in the north have given rise to uncertainties about the nation's stability after more than 14 years of consecutive warfare.

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US and Iran Work Together Against ISIS, This Time in Lebanon - Newsweek