Things just got a lot dicier in Syria as the military forces of Russia, Syria, and Iran warned the United States that any further military action would cross "a red line" and be met with a military response.

Somehow, I think when these guys talk about a "red line," they're being a helluva lot more serious than Obama was.

Daily Mail:

Russian and Iranian forces last night warned Donald Trump they would retaliate with military action if he launched more airstrikes on Syria.

In an ominous threat raising the prospect of war, they said the US President had crossed a 'red line' with his surprise missile bombardment on Bashar al-Assad's forces.

'From now on we will respond with force to any breach of red lines and America knows our ability to respond well,' the military chiefs said in a joint statement with militant group Hezbollah.

The Russian Embassy in London suggested on Sunday night there could be 'real war' if Moscow is presented with an ultimatum over Syria. 

But UK Foreign Secretary Boris Johnson will lead a push on Monday for Russia to face tough new sanctions unless it withdraws its support for Assad. 

At a G7 meeting in Italy, Johnson will call for Moscow to be threatened with isolation from the international community and a raft of economic punishments.

On a day of escalating tensions:

• The Kremlin mocked Britain for having 'no real influence on the course of international affairs' after Mr Johnson cancelled a planned trip to Moscow;
• UK's Defence Secretary Sir Michael Fallon said Russia was responsible for 'every civilian death' in Assad's chemical weapons attack on his own people;
• America warned Moscow it faces a 'relationship of competition and potential conflict' unless it stops supporting Assad's 'murderous regime';
• The White House told North Korea it should take US missile strikes against Syria as a warning that America is prepared to take military action against countries which break international agreements.

Despite the looming threat of war following the decision to launch airstrikes, Trump made his way on to his golf course in West Palm Beach, Florida on Sunday - marking his 16th visit to one of his own courses since he became president.

The U.S. has assets scattered all over the Middle East that would be vulnerable to an attack by Russia or, more likely, Hezb'allah.  Iran could even use Shia militias under its control in Iraq to target U.S. forces.  And, of course, there are a couple of thousand U.S. troops in Syria that are at risk if the Trump administration launches another strike at Syria.

Putin is no fool and doesn't want to force a confrontation with the U.S.  But can he trust his ally Assad not to seek out a confrontation with the U.S. by initiating another gas attack?  From Assad's point of view, a Russia-U.S. conflict means more Russian military assets sent to Syria.  But it also means that the U.S. would make a concerted effort to remove him from power. 

Russia has suspended its nominal cooperation with coalition air forces to avoid confrontations in the air.  In response, the U.S. has dialed back its bombing campaign against ISIS – a prudent move, considering the stakes.

In the end, it doesn't seem likely that Assad will risk such a confrontation, and President Trump won't attack unless his hand is forced.  But the battle lines have been drawn, and any kind of misunderstanding or miscalculation could lead to a war neither side wants.