What's Straining America's Relationship With Saudi Arabia

— -- As President Barack Obama departs on his fourth visit to the Kingdom of Saudi Arabia for a summit with Gulf Arab leaders, ABC News takes a look at some of the issues that have brought tensions between the old allies to an all-time high during this administration.

9/11

There has always been a great deal of scrutiny around what, if any role, Saudi Arabia may have played in the September 11, 2001, terror attacks.

But just weeks before the president's trip, new questions have arisen from current and former American lawmakers who allege that 28 never-before-seen pages of a 2002 Senate intelligence report on the attacks help demonstrate clear Saudi support for the al-Qaeda attackers.

Those officials say that they are working to declassify these documents and that they are key to understanding Saudi Arabia's network of support for the hijackers -- most of them Saudi nationals -- who entered the U.S. and managed to pull off the most complex and deadly terror attack in history.

Saudi Arabia has been angered deeply by the accusations and denies involvement, and said as far back as 2003 that it supports declassifying the 28 pages for all to see.

The White House says it's up to the intelligence community to declassify the documents.

Suing the Saudis

There is a bill on Congress right now -- which the administration opposes -- that is threatening to send relations into a tailspin.

Essentially, the bill would allow families of 9/11 victims to sue the Saudi government for compensation, which as of now is illegal. In response, the Saudis have threatened to sell off $750 billion in U.S. Treasury securities, a move that could have a severe impact on both economies.

Secretary of State John Kerry has said that creating this law could expose the U.S. to lawsuits and threaten the immunity of Americans overseas.

White House spokesman Josh Earnest spoke about this issue on Monday. “The whole notion of sovereign immunity is at stake,” he said. “The concern that we have is simply this: It could put United States, and our taxpayers, and our service members, and our diplomats at significant risk if other countries were to adopt a similar law.”

Iran

Perhaps no single issue has bothered the Saudi government more than the nuclear agreement between the U.S., six world powers, and Saudi Arabia's regional nemesis, Iran.

The nuclear agreement comes as Saudi Arabia and Iran have severed diplomatic relations over the execution of a Shia cleric in Saudi Arabia that led to protests and the destruction of the Saudi Embassy in Tehran.

The Saudi foreign minister suggested Iran will spend the billions of dollars freed up through the deal on "nefarious activities," such as its support for Houthi rebels in Yemen at war with the Saudi-backed government.

The U.S., meanwhile, is concerned that Saudi Arabia's relentless bombing campaign in Yemen may simply be pouring more fuel on the fire, leading to a resurgence of al-Qaeda fighters.

Oil

Saudi Arabia's oil sales to the U.S. are at their lowest level since the economic recession in 2009.

The U.S. has increased imports from Canada and discovered its own vast shale reserves. This has led to a drop in price on the global market, and although there is no dispute over the price of oil, it has long served as the most crucial issue of shared interest between the nations.

In search of a solution, the Saudis have becoming increasingly reliant on China to buy their oil.

Obama's Own Words

Last month, The Atlantic magazine published a detailed report on Obama's foreign policy legacy, in which the president is quoted offering various criticisms of the Saudis, ranging from their treatment of women, to their efforts to spread fundamentalist Islam in Indonesia, to their reliance on the U.S. military to solve regional conflicts.

The Saudis responded angrily to the president's suggestion that too many allies had become "free riders."

The Atlantic also reported that they Saudis were furious after President Obama pulled back from his "red line" in Syria and chose not to bomb the Assad regime following its use of chemical weapons.