(Reuters) - A deadly assault on an Iranian Revolutionary Guards parade dealt a stunning blow to Iran’s security establishment, which has often said it can repel any threat no matter how big, even from the United States and its chief Middle East ally Israel.

A general view of the attack during the military parade in Ahvaz, Iran September 22, 2018. Tasnim News Agency/via REUTERS

Saturday’s shooting attack, among the worst ever on the Guards, illustrated that Iran’s elite force, which answers directly to Supreme Leader Ayatollah Ali Khamenei, can be vulnerable to guerrilla-style operations.

Iran had enjoyed relative stability compared to Arab neighbors who have grappled with political and economic upheaval touched off by popular uprisings in 2011.

The Guards have vowed to retaliate for the attack.

WHO WAS RESPONSIBLE?

Iran blamed the United States and its Gulf Arab neighbors for the bloodshed. But it has presented no evidence.

An Iranian ethnic Arab opposition movement called the Ahvaz National Resistance, which seeks a separate state in oil-rich Khuzestan province, claimed responsibility for the attack.

So did Islamic State, who also claimed responsibility for a 2017 attack at the Iranian parliament and the mausoleum of Ayatollah Ruhollah Khomeini, the founder of the Islamic Republic, which left 18 dead.

Neither group has presented proof of their involvement.

Arab opposition groups have a long list of grievances against Iranian leaders and their frustrations are growing.

The city of Ahvaz, where the attack took place, is the capital of Khuzestan, a region bordering Iraq where the majority of Iran’s Arab minority lives.

The community has long felt neglected by the Persian-dominated central government in Tehran.

The area has been hit particularly hard by the economic problems afflicting the entire country and the unemployment rate in Khuzestan is 14.5 percent, higher than the national rate of 11.8 per cent.

Poor living standards have been compounded by electricity shortages and a severe drought, which locals blame on mismanagement by the central government. Residents of Ahvaz have been forced to stay inside their homes on some days because of severe sandstorms linked to drought in the past year.

Armed opposition groups have played on this discontent to attempt to drum up support for their actions which have included attacks on oil pipelines in the region. Civil rights activists say these violent attacks undermine their peaceful efforts to help the community and lead to widespread arrests.

The Kurds in western Iran and the Baluch in the southeast, both prominent minority groups, also complain of central government neglect. Armed Kurdish opposition groups have clashed with the Guards in the border area with Iraq in recent months, leading to several dead and wounded on both sides.

In early September, the Guards fired seven missiles at a base of a Kurdish opposition group in northern Iraq, killing at least 11 people.

WHAT HAS BEEN THE REACTION TO THE ATTACK AMONG IRAN’S RIVAL POLITICAL FACTIONS?

Such attacks tend to unite Iranian reformers and hardliners despite sharp differences over domestic and foreign policies.

President Hassan Rouhani has pushed back against the growing economic and political influence of the Guards in recent years but, after the Ahvaz attack, it will be difficult for Rouhani to challenge them.

The violence has led to a boost in support for the Guards, according to analysts, which they will likely use to silence their critics, including Rouhani.

President Donald Trump’s decision to withdraw the United States from an international nuclear deal from Iran will also give hardliners more political leverage because they argued that the United States should not be trusted.

HOW WILL THE REVOLUTIONARY GUARDS REACT?

Senior commanders have said the Ahvaz attack was carried out by militants trained by Gulf states and Israel and backed by America. But it is unlikely that the Guards will strike any of these foes directly.

They will probably present a show of strength by launching missiles at groups operating in Iraq or Syria that may be linked to the militants who carried out the attack.

After the Tehran attack by Islamic State in 2017, the Guards launched missiles at militant groups in eastern Syria days later. And after a series of clashes with Kurdish opposition groups in recent months, the Guards unleashed missiles at a Kurdish opposition base in northern Iraq in early September.

The Guards are also likely to enforce a tight security policy in Khuzestan province for the foreseeable future, arresting any perceived domestic opponents including civil rights activists.

(This version of the story was refiled to correct spelling of Khamenei in paragraph 2)