Imagine waking to the sound of your children choking from a chemical gas attack. My brother, a Syrian father living outside Damascus in 2013 at the height of the Syrian civil war, didn't have to imagine that gruesome scene -- he lived through it with his wife and four children. Thankfully, he and his family fled before they succumbed to the attack. When they returned, however, countless neighbors lay dead in their homes.

If Vladimir Putin's murderous crimes against the Ukrainian people are not stopped, the world could soon be witnessing such a monstrous chemical attack on a suburb, or Kyiv, Kharkiv, or Lviv. The U.S. and its allies would do well to remember that mass executions, similar to what we are seeing in Ukraine, took place in Syria when Putin and Syrian dictator Bashar al-Assad were not stopped in their assault on the pro-democracy, anti-Assad uprising that began in 2011.

Today, accounts of Putin's war crimes against Ukrainians flood the news -- including the bombing of hospitals, rape as a weapon of war, civilians left for dead, and the remains of mass executions.

Yet, sadly, this story isn't new. Since at least 2015, Putin's military has carried out ground operations in Syria and directly supported Assad's reign of terror with a brutal air campaign that, as in Ukraine, regularly targeted civilians.

Indeed, Putin clearly used the Syrian conflict as a testing ground for his ruthless military campaign in Ukraine. Since 2015, Russian planes repeatedly bombed civilian hospitals in Damascus, Aleppo, and many other Syrian cities -- a ruthless tactic now used shamelessly, and seemingly without fear of reprisals, in Ukraine. Another sickening tactic first employed by Russian troops in Syria is smoking out civilians from their residences with smoke bombs and shooting them as they flee, a common occurrence now in Ukraine.

How long will it be before we have substantiated reports of chemical gas attacks -- as we had in Syria nearly a decade ago, and which my brother's family survived -- in the towns and villages across Ukraine? We already have plenty of evidence of mass executions in Ukraine, similar to what we saw in a Damascus suburb in 2013 when video footage surfaced of a Syrian security officer leading prisoners to a mass grave, shooting them, and burning their bodies with gasoline.

The horrors perpetrated in Syria drew me and other supporters of freedom in Syria to lobby for, and see passed and signed into law, the 2019 Caesar Act. The act holds Assad's supporters — including Russia — accountable for their atrocities by expanding the U.S. government's ability to sanction individuals, businesses, and governments. But the Caesar Act is just one step forward.

The U.S. and its Western allies can help halt Russia's advances by providing Ukraine with defensive weapons such as tanks and combat aircraft. President Biden just signed a $40 billion package designed to provide $100 million per day to Ukraine in military equipment, food, and refugee assistance. The money will only last through September, but lawmakers would be wise to continue support until Putin is fully defeated.

If Putin is victorious, Syria and Ukraine won't be the last to suffer his horrific war tactics. The most vulnerable are the small Balkan nations of Moldova and Georgia. But Sweden too is vulnerable, due to its strategic location in the Baltic Sea. It's worth noting that the NATO states of Bulgaria, the Czech Republic, Estonia, Latvia, Poland, Romania, and Slovakia, all triggered NATO's Article 4, signaling they felt threatened by Russia's invasion of Ukraine.

Modern-day Syria is a chilling example of what could happen if the U.S. and other Western powers fail to defeat Putin entirely. Although the Syrian uprising began 11 years ago, families are still living through nightmarish conditions. In addition to the nearly 12 million Syrians displaced by the conflict, and more than 250,000 killed, around 100,000 people have simply disappeared, either arbitrarily imprisoned or executed.

If Putin is left unchecked, Ukrainians could live under hostile rule for years to come like their Syrian counterparts -- and NATO and non-NATO countries alike could be invaded next. Putin leads one of the largest militaries in the world, and if the West allows his sick crusade to continue, we may soon be witnessing a global humanitarian crisis not seen since World War II.

Dr. Tarek Kteleh is a practicing medical doctor, president of Rheumatology of Central Indiana, and a member of Citizens for a Secure and Safe America, a nongovernmental organization whose mission is to promote security in the Middle East and democratic progress in Syria. He is the author of The Six Pillars of Advocacy: Embrace Your Cause and Transform Lives.