This week is the thirteenth anniversary of 9/11, when al-Qaeda killed 3,000 people in New York, Washington and Shanksville, Pennsylvania.

Fifteen of the nineteen hijackers of the planes used in the strikes were Saudis.  So was al-Qaeda's leader, Usama bin Ladin.  Since then, the United States has applied considerable pressure on the kingdom to combat support for terrorism.

Saudi Arabia's record has been mixed.  A chain of dramatic al-Qaeda attacks in the kingdom itself a decade ago killed scores of people.  That experience drove Saudi Arabia to work vigorously to prevent attacks inside the country and pursue terrorists committed to them.

The kingdom has also blocked funds intended for terrorists, discouraged religious sermons supportive of terrorism, and detained and rehabilitated militants.  Yet over the years, the United States has complained -- for good reason -- that these undertakings have been constrained by Saudi capabilities as well as Saudi politics.

The country's political leadership (the al-Saud royal family) and religious leadership (adherents of the austere Wahhabi brand of Sunni Islam) have coexisted in a symbiotic relationship for more than two centuries.  They have depended on each other for support and legitimacy among the population.  Too much pressure from one on the other puts the power and influence of both groups at risk.

Today, one thing is clear: the Saudi political leadership views ISIS as a direct threat to the kingdom.  Nothing drove this home more than the discovery earlier this year of scores of Saudi citizens inside the kingdom, colluding with Saudi members of ISIS in Syria, to assassinate top security and religious officials in the country.

The discovery was followed by the first attack in five years on Saudi soil by al-Qaeda's Yemen branch (AQAP) - one of Saudi Arabia's most menacing threats.  In July, half-a-dozen Saudi AQAP members stormed a border post from the Yemeni side.  A couple of them made their way to a nearby border town and blew themselves up inside a government building.

Three weeks ago, AQAP declared its support for ISIS.  There are signs of ISIS-AQAP collaboration.  For Saudi Arabia, the ISIS terrorist threat is metastasizing.

To add insult to injury, some of the Saudi colluders with ISIS arrested earlier this year, and most of the Saudi border assailants in July, graduated from the kingdom's well-regarded terrorist rehabilitation program.

According to Saudi officials, one in ten of those who attend the program return to extremism, and two in ten of former Guantanamo Bay detainees who attend the program revert to militancy.  Non-Saudi figures may be much higher.  But even these numbers would be significant, as thousands have passed through the correctional center.

Ideological support for ISIS among Saudi citizens is thought not to be insignificant.

In face of this evolving threat, the Saudi political leadership has shown interest in dampening ideological, financial and jihadi support for ISIS, AQAP and other al-Qaeda-style groups at home; pursuing criminal procedures against these militants and their supporters; and using some soft power to discourage ISIS support by other Muslims outside the kingdom.  Saudi Arabia may be supplementing these overt measures with covert operations in the same vein, including in Iraq.

As Washington seeks to partner with Saudi Arabia in the fight against ISIS, it should neither dismiss an important role for the kingdom, nor harbor unrealistic expectations about the kingdom's participation in overt military, psychological and other operations.  The best approach to partnering with the Saudis is working with them to augment their recent activities that complement Washington's own interests.  We must take our cue from what the Saudis are showing they are willing and politically able to do.

This approach recognizes the benefits and limits of our strategic partnership with Saudi Arabia and the small Persian Gulf states: their political interests sometimes converge and at other times diverge from our own.

In this regard, it is important to be mindful of two important caveats.

The first is that, beyond the ISIS fight, it would be foolish to consider Saudi Arabia as a partner to help work toward a just political solution including a democratic character in Iraq and Syria.  Saudi Arabia is blatantly antipathetic to democratic agendas as well as Shia political power in the region.  The kingdom's effort to undermine more representative government in neighboring Bahrain is a case in point.  The Saudi leadership views democracy on its doorstep as a threat to its own absolute power at home.

The second regards the kingdom's understanding of terrorism.  When it comes to defining a terrorist, Saudi Arabia does not distinguish between deadly militants and nonviolent political activists.  The kingdom recently imprisoned a widely celebrated Saudi human rights advocate as a terrorist.

This issue comes into play as the United States considers helping to expand Saudi Arabia's counter-terrorism capabilities.  Washington must continue to work toward ensuring that enhanced technological and other capabilities are not employed against the wrong people.  This includes the very kind of people whose calls for basic political reform Washington favors.

Boghardt, a former U.S. intelligence analyst, is a fellow in Gulf politics at The Washington Institute.