Despite the historic nuclear agreement between the US and Iran, and talk of the rising political influence of Iran’s "moderates," who favor easing tensions with the US, the fact is that a core constituency in the Iranian regime remains vehemently opposed to warming relations with America.

That constituency is led by the Islamic Revolutionary Guards Corps (IRGC), the security and military organization that is responsible for the protection and survival of the regime. You can't really understand Iran without understanding the IRGC: why it was formed, what role it plays in Iran's foreign policy, and why it is so deeply anti-American.

Here, then, are six facts about the IRGC that help explain why Iran behaves the way it does — and why it's unlikely to change that behavior anytime soon.

1) The IRGC is not just a military — it also has huge political and economic power in Iran

In order to understand how the IRGC got to where it is, you need to know where it came from.

The 1979 Islamic Revolution in Iran was many things, but above all it was a reaction to American influence in Iran. Iran’s ruling monarch, Mohammad Reza Pahlavi, was considered a tyrant by the revolutionaries. They saw the shah’s oppressive policies as the bitter fruit of American influence. Armed and trained by America, Iran’s secret police and military were the blunt ends of that oppression.

The secret police was dissolved after the revolution, but the military was spared. Revolutionaries still didn’t trust the military. To curb its power and protect the revolution from a potential military coup, they formed the IRGC.

Since its establishment in the midst of the 1979 revolution, the IRGC has gradually grown to become a pillar of the Islamic Republic. It is the designated defender of the Islamic Revolution, the guardian of the supreme leader, and the chief mechanism of Iranian coercive power.

The IRGC has a massive commercial arm that is involved in industrial construction, shipping, telecommunications, and media. It oversees Iran’s sensitive covert operations and much of its foreign intelligence gathering, from Latin America to Southeast Asia.

It controls Iran’s most important strategic deterrent, its ballistic missile program, and facilitates relations with Iran’s closest regional allies and clients, such as Lebanese Hezbollah, Shia militias in Iraq, and the regime of Bashar al-Assad in Syria.

2) The IRGC sees itself as the guardian of Iran’s Islamic Revolution and the supreme leader

The IRGC, whose name in Persian means "Army of the Guardians of the Islamic Revolution," was started by loyalists of Ayatollah Ruhollah Khomeini — the founder of the Islamic Republic and its first supreme leader. Its charge was to safeguard the revolution and Iran’s new governmental system of clerical rule.

The supreme leader was the heart of that system and its highest authority. Above all else, defending the revolution meant defending the supreme leader and the primacy of his rule.

From Khomeini to his successor, Ali Khamenei, Iran’s two supreme leaders have benefited from the IRGC’s support. The IRGC’s coercive power has underwritten the domestic policies and foreign ambitions of the supreme leader’s office.

Unlike Khomeini, who had a large following before the revolution, Ali Khamenei (Iran’s current supreme leader) lacked a natural constituency when he took power. The IRGC filled that void, and forged a tight, symbiotic relationship with the supreme leader. The IRGC has vigorously supported Khamenei’s policies, and in return he has preserved the IRGC’s unique and dominant position.

This doesn’t mean the IRGC gets whatever it wants. The supreme leader rules by balancing the interests of Iran’s main stakeholders — the elected government, the parliament, the clergy, the electorate, and the IRGC.

However, because of the IRGC’s proximity to the supreme leader and staunch advocacy for his station, the organization gets what it wants more often than not.

3) The IRGC’s aims are a mix of ideology and national self-interest

Safeguarding the role of the supreme leader and the centrality of Islam in Iran are the IRGC’s top priorities. To that end, the IRGC wants to preserve its status and political influence, because it believes itself to be the only organization in Iran that can truly defend the country’s Islamic system of governance.

The IRGC sees Western influence as the revolution’s fiercest challenge. Democracy is viewed as a problematic outgrowth of Western influence, which is why the IRGC fears pro-democratic populist movements that aren’t outwardly supportive of the regime and conservative Islamic values.

To the IRGC, the intrusion of Western influence is a form of cultural warfare that it must fight at every turn. Within Iran, it does this mainly through its massive volunteer militia, known as the Basij. Local branches of Basij exist at every level of schooling (primary through university), in all government offices and business, and in neighborhood mosques throughout Iran.

At one end of the spectrum, the Basij operates like an Iranian version of the Boy Scouts or Girl Scouts, in that its aim is to develop young people into proper, moral citizens who are religiously devout and devoted to the supreme leader.

At the other end, the Basij operates more like a political pressure group. The Basij has used violence and coercion to disrupt pro-democratic popular movements, combat political unrest, and promote a rigid social order. During the 2009 protests, for example, it was blamed for some of the worst violence, including the killing and rape of scores of protestors.

Outside Iran, the IRGC’s mission is more complex. It wants to purge the Middle East of American influence, which means both compelling the retreat of American forces from the Persian Gulf and seeking the defeat of the state of Israel. It is also deeply mistrustful of Iran’s Arab neighbors — especially Saudi Arabia — for their close ties to Washington.

Riyadh’s pro-American stance, and its promotion of a virulently anti-Shia form of Sunni Islam, underpins Iran’s tensions with the Saudi kingdom. As those tensions have helped fuel Saudi and Iranian involvement in regional wars (in Syria, Iraq, and Yemen), the IRGC has become a central node in a simmering Iranian-Saudi conflict.

To rid the Middle East of American influence, and to counter Saudi’s regional interests, the IRGC relies on a robust network of client armed groups. Hezbollah, Iraqi Shia militias, various Palestinian groups (including Hamas), and Yemen’s Ansarallah (also known as the Houthis) are all backed by the IRGC to further Iranian interests across the region.

Nowhere is this more apparent than in Syria’s civil war, where the IRGC has enlisted foreign militants and its own soldiers to defend the Assad regime against the largely Sunni rebellion backed by Saudi Arabia, Qatar, Turkey, and the United States.

4) The bitter experience of the Iran-Iraq war — and US support for Iraq during it — still looms large for the IRGC

The IRGC was a ragtag militia manned by poorly trained, under-armed recruits when Saddam Hussein invaded Iran in September 1980. The Iran-Iraq war lasted nearly eight years and devastated Iran. Hundreds of thousands of Iranian troops were killed.

The United States supported Iraq during the war, which Iran saw as an American plot to destroy the fledgling revolution. Iran blames America for providing intelligence on Iranian positions to the Iraqis, enabling Saddam’s use of chemical weapons, and pressuring Iran through the use of US naval power in the Persian Gulf.

The downing of Iran Air Flight 655 by the USS Vincennes, which killed 290 civilians, is still fresh in the minds of Iran’s war veterans. Regardless of how the accident occurred, to the IRGC and Iran’s leadership it was an act of war, and a harbinger of escalations to come if the war continued. Unable to fight both Iraq and the US, Iran’s leadership felt compelled to end the war with Saddam.

Iran’s Arab neighbors — with the exception of Syria — all supported Saddam as well. Saudi Arabia and Kuwait, both American allies, provided much of the financing for Iraq’s war effort. The IRGC — and Iran’s leadership more broadly — holds a deep grudge against the United States, Saudi Arabia, and others as a result of their support for Saddam Hussein.

The vast majority of the IRGC’s current leadership got their start as volunteers in the Iran-Iraq war. They bear all the scars of that conflict, but also the pride and confidence of having survived it. The IRGC and its Basij popular militia wing lost more lives than any other force during the war — a fact that has not been lost on them.

Since the end of the war, the IRGC has formed its politics to ensure that the sacrifices of the war veterans are not overlooked and that the causes they fought for (the preservation of the revolution and its Islamic ideals) are not compromised by things such as the spread of popular democracy, the weakening of social mores, and the normalization of relations with America.

5) The IRGC sees sinister motives behind nearly all US actions in the Middle East

There was a brief opening after 9/11 where the IRGC flirted with cooperating with the United States. Qassem Suleimani, the commander of the IRGC’s covert special operations wing known as the Quds (Jerusalem) Force, greenlit limited intelligence sharing with US diplomats in the buildup to the US invasion of Afghanistan. The IRGC had almost gone to war with the Taliban in 1998, and was somewhat open to assisting America’s war on their mutual enemy.

The collaboration was short-lived.

President George W. Bush’s "axis of evil" speech tainted the budding relationship, and was seen as a direct betrayal by Suleimani. It was confirmation to the IRGC and the supreme leader that the United States would never change its stance toward the Islamic Republic, and could never be trusted.

Instead of cooperating with the US, the IRGC worked to undermine the US occupations of Afghanistan and Iraq. The IRGC helped train and arm anti-American Afghan militants, and led a covert ground war against US influence in Iraq. The IRGC’s control of Iraqi militias and influence over Iraqi politicians partly led to the departure of US forces from the country.

The history of mistrust similarly underlies the IRGC’s understanding of US policies in Syria and Iraq today. From the IRGC’s perspective, the US war against ISIS is a smokescreen designed to conceal America’s efforts to uproot Iranian influence in the Levant by toppling Iran’s longstanding ally, the Assad regime, weakening Lebanese Hezbollah, and destabilizing the region to create a pretext for the return of US forces to Iraq.

6) The IRGC matters more to US policy than most care to admit

The IRGC is an inexorable part of Iranian foreign and strategic policy. It is second only to the supreme leader in its influence over the strategic arena and plays an outsize role in foreign policy through its relationships and on-the-ground investments.

The supreme leader sits atop Iranian foreign and strategic policy, and sets guidelines for both. Under the leader, the elected government takes the lead in managing foreign policy. Similarly, strategic policy is the purview of the Supreme National Security Council (on which the president, the IRGC commander, and several other institutional heads sit).

The government has a lot of influence over policy. It runs the diplomatic corps and sets the tone for Iran’s foreign policy writ large. However, in Iran’s most important relationships — in Lebanon, Iraq, and Syria — the IRGC sets the parameters for Iranian policy. This is because the heart of Iran’s influence in those countries is the IRGC’s relationships with foreign leaders and client armed groups.

The US government interacts with Iran’s elected government, which serves as the face of the Islamic Republic in international affairs. This gives the appearance that the government’s policy preferences are backed by the whole of the regime, but often they are not. The government speaks for the regime in official discussions, but outside of negotiations like the Iran deal, the government’s predilections are not necessarily shared by the IRGC.

The issues that are of central concern to Washington and Iran’s neighbors are generally those dominated by the IRGC. From the ballistic missile program to Iranian activities across the Middle East and beyond, the IRGC controls much of the strategic space that worries the West and Iran’s rivals the most.

The supreme leader maintains ultimate authority over strategic and foreign policies, including covert activities, but he cedes much of the decision-making to the IRGC so long as its decisions are not deemed harmful to Iran’s broader interests. Although the supreme leader and IRGC disagree at times, they are in agreement more often than not.

This is a challenge to the US because the IRGC is essential to all of the regional issues that Washington is currently deeply engaged in (Syria, Iraq, Yemen, Israel) and to regional security in general.

In order for US-Iran relations to ever transcend mutual antagonism, the IRGC will need to either abandon the investments and activities that have made it so formidable or move beyond its fundamental anti-Americanism. That is unlikely to happen. What the future holds is less likely to be a US-Iranian rapprochement than the continued management of tensions and disagreements.

The nuclear deal opened the door for the Iran’s return from isolation. More than ever before, Iran is in control of its own destiny. If it wants peace and prosperity, it will need to rethink its priorities — starting with those of the IRGC.

Afshon Ostovar is the author of the new book Vanguard of the Imam: Religion, Politics, and Iran’s Revolutionary Guards. He is a senior analyst at the Center for Strategic Studies at CNA, and will be moving on to the Naval Postgraduate School in the summer.