A Russian diplomat speaking to Hezb'allah TV said Russia will shoot down any U.S. missiles fired at Syrian targets and then attack the "sources where the missiles were fired."

The threat of retaliation by Russia changes the calculations in U.S. plans to respond to the poison gas attack on civilians over the weekend, presumably carried out by Syria.

NBCNews:

"If there is a strike by the Americans, then we refer to the statements of President [Vladimir] Putin and the chief of staff that the missiles will be downed and even the sources from which the missiles were fired," Russian Ambassador to Lebanon Alexander Zasypkin told Hezbollah's al-Manar TV.

The envoy's comments came after the U.S. heaped condemnation on Syria and its main backer Russia since images emerged of the suspected poison gas attack that allegedly killed dozens in Douma on Saturday.

Trump canceled a trip to Latin America to focus on the Syria incident, the White House said.  Defense Secretary Jim Mattis also canceled plans to travel to California in the coming days.  Trump told reporters all options were on the table regarding Syria.

The USS Donald Cook, a Navy destroyer, left a port in Larnaca, Cyprus on Monday.  The guided missile destroyer is armed with Tomahawk cruise missiles, which were used a year ago after an alleged sarin gas attack on Syrian civilians.

Russia has sophisticated anti-ship missiles that would make short work of the Cook.  But some analysts believe that Trump is going to do much more than simply lob a few cruise missiles at Syrian targets.

Eurocontrol, the pan-European air traffic control agency, warned airlines Tuesday to exercise caution in the eastern Mediterranean due to possible airstrikes in the next 72 hours.

Retired Adm. James Stavridis, a former head of NATO and an NBC News analyst, warned that any U.S. strike on Syria would likely require manned aircraft and characterized it as a "high-risk operation."

"Last year was about sending a signal," Stavridis said, referring to the April 2017 strike ordered by Trump.  "This year its [sic] about destroying actual Syrian capability."

To destroy "Syrian capability," we are going to have to destroy the Syrian air force.  We may not know where all of Assad's chemical weapons are located, but we know where his planes are.  Apparently, Russia is thinking the same thing and will not allow that to happen.

How seriously should we take this threat?  We can't afford not to take it seriously.  We should be watching to see if Russia begins moving military assets to the expected targets to deter the U.S. – and especially France and Great Britain – from killing a lot of Russians in the strikes.  Putin will seek to divide the allies over the severity of the strikes, as France and Great Britain may not have the stomach for a confrontation with Russia.

You have to believe that Putin is itching for a chance to humiliate the U.S.  Sanctions over Crimea and Ukraine have hurt the Putin regime, and the West's constant criticism of human rights in Russia has Putin seething. 

But how far would he go to exact his revenge?  If Putin decides on a major confrontation in the Middle East, it could set off a series of regional conflicts in the region that would be hard to contain.  Also, war in the Middle East would drive up the price of oil worldwide, filling Putin's coffers and alleviating some of the pain from sanctions. 

Do you trust both Putin and Trump to keep a lid on any confrontation and not allow it to spiral out of control?