Washington’s interest in the Syrian civil war is partly about ensuring Israel’s regional security and partly it is about doing the bidding of America’s regional Sunni allies: Turkey, Jordan and the Gulf Arab States.
Saudi Arabia which has been vying for power as the leader of the Sunni bloc against the Shi’a-dominated Iran in the regional geopolitics was staunchly against the invasion of Iraq by the Bush Administration in 2003.
The Baathist regime of Saddam Hussein constituted a Sunni Arab bulwark against the Iranian influence in the Arab World. But after Saddam was ousted from power in 2003 and subsequently when elections were held in Iraq which were swept by the Shi’a-dominated parties, Iraq has now been led by a Shi’a-majority government that has become a steadfast regional ally of Iran. Consequently, Iran’s sphere of influence now extends all the way from territorially-contiguous Iraq and Syria to Lebanon and the Mediterranean coast.
The Saudi royal family was resentful of Iranian encroachment on traditional Arab heartland. Therefore, when protests broke out against the Assad regime in Syria in the wake of Arab Spring uprisings of 2011, the Gulf Arab States along with their regional allies, Turkey and Jordan, and the Western patrons gradually militarized the protests to dismantle the Iranian resistance axis.
More to the point, the United States Defense Intelligence Agency’s declassified report  of 2012 clearly spelled out the imminent rise of a Salafist principality in northeastern Syria in the event of an outbreak of a civil war in Syria. Under pressure from the Zionist lobby in Washington, however, the Obama Administration deliberately suppressed the report and also overlooked the view in general that a proxy war in Syria will give birth to radical Islamic jihadists.
The hawks in Washington were fully aware of the consequences of their actions in Syria, but they kept pursuing the ill-fated policy of nurturing militants in the training camps located in the border regions of Turkey and Jordan to weaken the Baathist regime in Syria.
The single biggest threat to Israel’s regional security was posed by the Shi’a resistance axis, which is comprised of Iran, the Assad regime in Syria and their Lebanon-based proxy, Hezbollah. During the course of 2006 Lebanon War, Hezbollah fired hundreds of rockets into northern Israel; and Israel’s defense community realized for the first time the nature of threat that Hezbollah and its patrons, Iran and the Assad regime in Syria, posed to Israel’s regional security.
Those were only unguided rockets but it was a wakeup call for Israel’s military strategists that what will happen if Iran passed the guided missile technology to Hezbollah whose area of operations lies very close to the northern borders of Israel?
Regarding the Western interest in collaborating with the Gulf Arab States against their regional rivals, bear in mind that in April last year, the Saudi foreign minister threatened  that the Saudi kingdom would sell up to $750 billion in treasury securities and other assets if Congress passed a bill that would allow Americans to sue the Saudi government in the United States courts for its role in the September 11, 2001 terror attack.
Moreover, $750 billion is only the Saudi investment in the United States, if we add its investment in the Western Europe and the investments of UAE, Kuwait and Qatar in the Western economies, the sum total would amount to trillions of dollars of Gulf’s investments in North America and Western Europe.
Furthermore, in order to bring home the significance of Persian Gulf’s oil in the energy-starved industrialized world, here are a few rough stats from the OPEC data: Saudi Arabia has the world’s largest proven crude oil reserves of 265 billion barrels and its daily oil production exceeds 10 million barrels; Iran and Iraq, each, has 150 billion barrels reserves and has the capacity to produce 5 million barrels per day, each; while UAE and Kuwait, each, has 100 billion barrels reserves and produces 3 million barrels per day, each; thus, all the littoral states of the Persian Gulf, together, hold more than half of world’s 1500 billion barrels of proven crude oil reserves.
Additionally, regarding the Western defense production industry’s sales of arms to the Gulf Arab States, a report  authored by William Hartung of the US-based Center for International Policy found that the Obama Administration had offered Saudi Arabia more than $115 billion in weapons, military equipment and training during its eight years tenure.
Similarly, the top items in Trump’s agenda for his maiden visit to Saudi Arabia in May were: first, he threw his weight behind the idea of Saudi-led “Arab NATO” to counter Iran’s influence in the region; and second, he announced an unprecedented arms package for Saudi Arabia. The package included between $98 billion and $128 billion in arms sales, and over a period of 10 years, total sales could reach $350 billion.
Thus, keeping the economic dependence of the Western countries on the Gulf Arab States in mind during the times of global recession when most of manufacturing has been outsourced to China, it is unsurprising that when the late King Abdullah of Saudi Arabia decided to provide training and arms to Sunni Arab jihadists in the border regions of Turkey and Jordan against the Shi’a-dominated regime of Bashar al-Assad in Syria, the Obama Administration was left with no other choice but to toe the destructive policy of its regional Middle Eastern allies, despite the sectarian nature of the proxy war and its attendant consequences of breeding a new generation of Islamic jihadists who would become a long-term security risk not only to the Middle East but to the Western countries, as well.
Similarly, when King Abdullah’s successor, King Salman, decided to invade Yemen in March 2015, once again, the Obama Administration had to yield to the dictates of Saudi Arabia and UAE by fully coordinating the Gulf-led military campaign in Yemen not only by providing intelligence, planning and logistical support but also by selling billions of dollars’ worth of arms and ammunition to the Gulf Arab States during the conflict.
Regarding the Pax Americana which is the reality of the contemporary global political and economic order, according to a recent infographic  by the New York Times, 210,000 US military personnel are currently stationed all over the world; including 79,000 in Europe, 45,000 in Japan, 28,500 in South Korea and 36,000 in the Middle East (of which, 28,000 have been deployed in the Persian Gulf alone, including 11,000 in the sprawling Al-Udeid airbase in Qatar).
By comparison, the number of US troops in Afghanistan is only 8,500 which is regarded as an occupied country. Thus, the Gulf Arab principalities are not sovereign states, as such, but the virtual protectorates of the corporate America.
In this reciprocal relationship, the US provides security to the ruling families of the Gulf Arab states by providing weapons and troops; and in return, the Gulf’s petro-sheikhs contribute substantial investments to the tune of trillions of dollars in the Western economies.