Sheldon Richman has an interesting article here
(http://www.informationclearinghouse.info/article43597.htm) entitled "Why Assad Isn't Our Son of a Bitch"
The statement "He may be a son of a bitch, but he's our son a bitch" is attributed to Franklin D. Roosevelt (FDR) in 1939 referring to Anastasio Somoza dictator of Nicaragua, and the first of three Somozas to rule that country. Of course the US has a long record of supporting brutal dictators (our sons of bitches) often after having helped install them into power. In Latin America they are too myriad to list but they go back to the nineteenth century with Porfirio Diaz in Mexico and later including Pinochet in Chile, Noriega in Panama, Batista in Cuba and Papa Doc Duvalier in Haiti. In Asia, Thailand, Vietnam, Pakistan and the Philippines have all been ruled by US backed dictators. In the Middle East of course there have been Saddam Hussein, Ali Abdullah Saleh (in Yemen) and Hosni Mubarak and now Abdel Fattah el-Sissi in Egypt. And once upon a time, Bashar al-Assad, like his father Hafez al-Assad was one of "our sons of bitches".
If you recall, in 1991 Assad pere (Hafez) backed the US-led coalition against Saddam Hussein in the First Gulf War, offering 100,000 troops. Syria was a very useful ally for the Coalition, because the support of a major Arab nation helped to lend the whole enterprise legitimacy in the Arab world. Then following 911, Assad fils (Bashar) offered "intelligence" cooperation with the US and its allies. What this meant in practice was that Syria acted as a contractor for US intelligence - offering torture services for which the CIA or other agencies wanted a cutout. Ask Meir Arar, a Canadian of Syrian birth, who was kidnapped while passing through JFK in New York and "rendered" to Syria where he was imprisoned and tortured.
So for a long time the Assads were "our sons of bitches". But this changed in 2011 with the "Arab Spring". Violence broke out at one or more demonstrations. Who started it is not entirely clear, but the government responded with a very strong hand. Soon al-Assad found he was the subject of sanctions by the US Government. Other governments rapidly followed suit.
An important question is why did al-Assad go from being "our son of a bitch" to a pariah? This was before the Syrian Army had started its arial campaign using barrel bombs and the like. Was it simply because President Obama was a man of principle and champion of democracy and could no longer stand the brutality of dictators like al-Assad? Hardly likely given the support it still lends to odious regimes such as that of el-Sissi in Egypt, or the royal family in Saudi Arabia.
In his article Richman suggests that the answer is Iran. After the disaster of the Iraq War, Shiite Iran's influence in the region waxed considerably. This caused considerable concern to the major Sunni powers, Turkey, Saudi Arabia and the Gulf States. They feared the emergence of a "Shia Crescent", running from Iran through Iraq and Syria to south Lebanon where Hezbollah has its roots. I think this is one reason for the US dropping al-Assad, but I think the real reason lies with Israel.
Israel has long planned for the overthrow of any rival power in the Middle East. This has all been spelled out in the document 1996 A Clean Break: A New Strategy for Securing the Realm, commissioned by Benjamin Netanyahu. Among other things it called for the removal by force of Saddam Hussein and the destabilization of Syria. With the the help of the Israeli fifth column of Neocons in the US security establishment President G. W. Bush was persuaded that his manhood demanded the invasion of Iraq and the removal of Saddam Hussein. If that had gone successfully no doubt he would have been persuaded by the mantra "Real Men go to Tehran". But it didn't work out. At least not for the United States. Iran became stronger as a result of a Shiite government taking power in Bahgdad.
Furthermore Israel was humiliated by Hezbollah when it invaded Lebanon (again) in 2006. The Shiite Hezbollah receives armaments from Iran (via Syria). Israel clearly wanted to cut off this supply route and deal a stinging blow to Iran.
The best opportunity for Israel to hit at Iran was by destroying the al-Assad regime in Syria. Not only would it be one in the eye for the Iranian regime, it would also cut off Hezbollah from its weapons supply. So with the help of its faithful Neocon friends in Washington, Israel was able to persuade the Obama administration that it was time to cut Assad loose. Obama was locked in a bitter struggle with Netanyahu over the nuclear deal with Iran, and probably felt he didn't want another conflict with Israel on his hands, especially since he was facing re-election. So the US ditched its faithful son of a bitch and backed the opposition in Syria, even to the point of backing very nasty jihadi groups like Al Nusra (Al Qaeda in Syria)
I can see no reason why Iran should be considered a real threat to the USA. But I think the following comment from Andrew Sullivan (on NeoConservatism) explains why it acts that way and wants al-Assad gone.
The closer you examine it, the clearer it is that neoconservatism, in large part, is simply about enabling the most irredentist elements in Israel and sustaining a permanent war against anyone or any country who disagrees with the Israeli right. That's the conclusion I've been forced to these last few years. And to insist that America adopt exactly the same constant-war-as-survival that Israelis have been slowly forced into... But America is not Israel. And once that distinction is made, much of the neoconservative ideology collapses.
The results of NeoCon policies have been nothing short of a disaster on an epic scale. I predict that one day historians will not be kind in their assessment of the policies and actions of this bunch of immoral warmongers.