On Saturday, the U.S., U.K., and France launched airstrikes on the Assad regime's alleged chemical weapons facilities in Syria.
The Prime Minister will say the UK joined the United States and France in co-ordinated strikes following the chemical weapons attack in Douma to "alleviate further humanitarian suffering".
The Prime Minister will tell MPs on Monday that the strikes were in the national interest because the use of chemical weapons can not be normalised, including in the UK.
"The fact that the atrocity can take place in the world today is a stain on humanity", she said.
The Assad regime has repeatedly used chemical weapons, including chlorine and nerve agents, during the seven-year war, despite a 2013 deal brokered by the United States and Russian Federation to remove Syria's chemical weapons.
"We are confident in our own assessment that the Syrian regime was highly likely responsible for this attack and that its persistent pattern of behaviour meant that it was highly likely to continue using chemical weapons", Mrs May will add.
The Western military response comes as the Syrian regime, backed by Russian Federation and Iran, has largely turned the war in its favor, retaking large swaths of territory from rebels since Russian Federation intervened in 2015.
Damascus and Moscow have vehemently denied that any chemical weapons were used in Douma and alleged instead that grim videos showing civilians foaming at the mouth after the attack were staged.
The bombings marked the biggest intervention by Western countries against Assad and ally Russian Federation.
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Both Russia and Iran decried what they called a violation of worldwide law and Syria's sovereignty, but it was unclear what, if any, response they would make.
The strikes have ratcheted up global tension, as the USA and Russian Federation exchanged threats of retaliation.
"There are certainly various asymmetric means through which Russia, Syria and Iran could respond inside Syria or outside", said Heller.
The rally in Omayyad Square in Syria's capital Damascus was described on state TV as a "salute to the achievements of the Arab Syrian Army".
According to Heller, "there are ways the Syrian government and its allies can make it more hard or less tenable for the United States and its coalition to operate inside Syria", including possibly Russian Federation using its air defenses to limit U.S. airpower.
Germany, Italy, Canada, Australia and European Council President Donald Tusk have "all have expressed their support for the actions that Britain, France and America have taken", the PM will add. "If al-Assad and his allies can weaken the SDF, they can crack the foundation of the U.S. military's planning for Syria".
In a telephone conversation with his Iranian counterpart, Hassan Rouhani, Putin and Rouhani agreed that the Western strikes had damaged the chances of achieving a political resolution in the seven-year Syria conflict, according to a Kremlin statement.
In an unusual move, the government says it will seek an emergency House of Commons debate on the airstrikes so legislators can have their say.