Absent the United States in the Persian Gulf, Iran would have been the most powerful regional military power. In addition, the Saudis have a substantial Shiite minority concentrated in the country's oil-rich east. The Iranians, also Shia, had a potential affinity with them, and thereby the power to cause unrest in Saudi Arabia.
Until this agreement with Iran, the United States had an unhedged commitment to protect Saudi Arabia from the Iranians. Given the recent deal, and potential follow-on deals, this commitment becomes increasingly hedged. The problem from the Saudi point of view is that while there was a wide ideological gulf between the United States and Iran, there was little in the way of substantial issues separating Washington from Tehran. The United States did not want Iran to develop nuclear weapons. The Iranians didn't want the United States hindering Iran's economic development. The fact was that getting a nuclear weapon was not a fundamental Iranian interest, and crippling Iran's economy was not a fundamental interest to the United States absent an Iranian nuclear program.
If the United States and Iran can agree on this quid pro quo, the basic issues are settled. And there is something drawing them together. The Iranians want investment in their oil sector and other parts of their economy. American oil companies would love to invest in Iran, as would other U.S. businesses. As the core issue separating the two countries dissolves, and economic relations open up -- a step that almost by definition will form part of a final agreement -- mutual interests will appear.
There are other significant political issues that can't be publicly addressed. The United States wants Iran to temper its support for Hezbollah's militancy, and guarantee it will not support terrorism. The Iranians want guarantees that Iraq will not develop an anti-Iranian government, and that the United States will work to prevent this. (Iran's memories of its war with Iraq run deep.) The Iranians will also want American guarantees that Washington will not support anti-Iranian forces based in Iraq.
From the Saudi point of view, Iranian demands regarding Iraq will be of greatest concern. Agreements or not, it does not want a pro-Iranian Shiite state on its northern border. Riyadh has been funding Sunni fighters throughout the region against Shiite fighters in a proxy war with Iran. Any agreement by the Americans to respect Iranian interests in Iraq would represent a threat to Saudi Arabia.
The View from Israel
From the Israeli point of view, there are two threats from Iran. One is the nuclear program. The other is Iranian support not only for Hezbollah but also for Hamas and other groups in the region. Iran is far from Israel and poses no conventional military threat. The Israelis would be delighted if Iran gave up its nuclear program in some verifiable way, simply because they themselves have no reliable means to destroy that program militarily. What the Israelis don't want to see is the United States and Iran making deals on their side issues, especially the political ones that really matter to Israel.
The Israelis have more room to maneuver than the Saudis do. Israel can live with a pro-Iranian Iraq. The Saudis can't; from their point of view, it is only a matter of time before Iranian power starts to encroach on their sphere of influence. The Saudis can't live with an Iranian-supported Hezbollah. The Israelis can and have, but don't want to; the issue is less fundamental to the Israelis than Iraq is to the Saudis.
But in the end, this is not the problem that the Saudis and Israelis have. Their problem is that both depend on the United States for their national security. Neither country can permanently exist in a region filled with dangers without the United States as a guarantor. Israel needs access to American military equipment that it can't build itself, like fighter aircraft. Saudi Arabia needs to have American troops available as the ultimate guarantor of their security, as they were in 1990. Israel and Saudi Arabia have been the two countries with the greatest influence in Washington. As this agreement shows, that is no longer the case. Both together weren't strong enough to block this agreement. What frightens them the most about this agreement is that fact. If the foundation of their national security is the American commitment to them, then the inability to influence Washington is a threat to their national security.
