Earlier today, gunmen attacked the Iranian Parliament and the shrine of Ayatollah Khomeini in Tehran, Iran, killing at least 12 people and wounding many others.

Don't be deceived by the relatively low casualty figures -- this attack is a big deal. The Iranian Parliament is Iran's valued pretense to Islamic democracy. And Khomeini is the father of the Iranian revolution. He made Iran what it is today. This is was a strike at the heart of the Shia-rooted Islamic Republic.

A familiar organization appears to be responsible. The Islamic State has published videos online which show its operatives running around the Parliament, so its claim to responsibility appears genuine. The nature of the assault also lends credibility to ISIS responsibility. After all, ISIS detests Shiites as heretics, specifically the sect's veneration of martyrs and religious leaders. They believe that visual homage to these individuals is an affront to Allah's singular glory. And this action serves two further purposes for ISIS.

First, it generates support for the ISIS cause. It tells prospective recruits, other jihadist groups, and fundraisers in the Islamic world that ISIS is serious about purging Shia Islam from the planet. For Sunni Salafi-extremists, destroying Shia Islam is as glorious an ambition as destroying the west.

The attack also allows ISIS to discredit Iran's government. By storming into two of Iran's most important institutions, ISIS has struck at the heart of its mortal enemy. It has also embarrassed Iran. According to the New York Times, the security force response was clearly inadequate: "[O]ne attacker left the Parliament an hour into the siege, then ran around shooting on Tehran's streets before returning to the building." In many ways, this attack is an ISIS version of the Doolittle raid, the first attack on Iranian soil. It sends a message to disheartened ISIS forces in Mosul and Raqqah that their cause is not lost.

Still, that's just one side of the coin, because Iran is likely to respond.

For a start, expect escalated tensions with Saudi Arabia. The Iranians regard the Saudis as the source of all Sunni-rooted jihadism. But the scale of this attack changes the game. Especially in the perceptions of the Revolutionary Guard-aligned hardliners. Those elements of the Iranian polity are fixated on acquiring a dominant strategic position over Saudi Arabia. They believe that to maintain a regional balance of power in Iran's favor, Saudi Arabia must fear Iran more than Iran fears it. Again, it doesn't really matter that Saudi Arabia had nothing to do with what happened. It simply matters how the Iranian regime perceives this attack. Because that perception may lead to Iranian terrorism against Saudi interests.

This is not a peripheral concern for the United States. Consider the 2011 Revolutionary Guard bomb plot against the then Saudi Ambassador (now foreign minister) to the United States. The Ambassador was to be targeted while dining at a Washington, D.C. restaurant. Unfortunately for the Iranians, they happened to hire an undercover DEA source to do the bombing. That gave us some insight into how the Iranians conduct their covert action strategy. When the DEA source told his Iranian handler that civilians would also die with the Ambassador, he responded "They want that guy done, if the hundred go with him f**k em." When told the restaurant was popular with U.S. Senators, the IRGC officer added "no big deal."

My point here is simple. The Iranian hardliners see ISIS as an extension of Saudi Arabia. It is likely that they will plot retaliation against Riyadh.

That risk is also exacerbated by the impact of these attacks on the more-moderate bloc of Iranian politics. Led by President Hassan Rouhani, these politicians are attempting to walk a tightrope between better relations with the west (especially European businesses), and placating the hardliners. But this significant attack will make it hard for Rouhani and co., to reject tougher domestic security restrictions and a more aggressive foreign policy.

There's one final issue that affects this situation. Ayatollah Khamenei's ill-health. The Supreme Leader holds the keys to power, but is expected to die within the next two years. Iran's various political factions know this. And they know that if their chosen candidate wins, they will have a lot more power to affect the nation's future course. Correspondingly, this attack will encourage the hardliners to take action to delegitimize more-moderate contenders to replace Khamenei.

As I say, today's events are a big deal.

(More of my thoughts on Iran's Revolutionary Guard here)