After days of hinting at military action in response to a chemical weapons attack on civilians in Syria, President Trump made that threat explicit on Wednesday morning in a message aimed at Russia:

Why it matters: Trump is denouncing Russia in a way he never has previously — and foreshadowing possible military action on Twitter.

Yes, but: In a follow-up tweet, he wrote: "Our relationship with Russia is worse now than it has ever been, and that includes the Cold War." But he also offered a carrot: the U.S. could help Russia economically if Russia would help in foreign policy. "Stop the arms race?" he added.

Between the lines: Russia has said it will retaliate against any strike that risks Russian lives, so the biggest remaining question is whether Trump limits his response to Assad regime targets or hits all three actors he has said share responsibility for the attack — Assad, Iran and Russia.

3 options for striking in Syria

Pinprick strikes on the Assad regime, like those he ordered last April after a previous chemical attack. The downside: Those strikes failed to deter Assad. More damaging strikes targeted at the Assad regime, for example striking "numerous regime airfields and military bases, and warning the Russians in advance," says Jennifer Cafarella of the Institute for the Study of War. The downside: "It will not harm Assad's backers and therefore is unlikely to weaken his resolve." Strikes that would affect all three actors Trump named, hitting targets like joint Russian-Iranian bases or command and control centers. The downside: Russia has said it will retaliate to strikes that endanger Russian troops.

The bottom line: Cafarella says if Trump chooses option number 3, Russia, Iran, and Assad might limit their response to attempting to shoot down the U.S. missiles or aircraft. More dangerous is the possibility of a counter attack, perhaps on a U.S. warship in the Mediterranean.

An improvised "vow"?

Meduza provides the context behind President Trump's tweet:

"Russia’s ambassador to Lebanon has unveiled what appears to be a major policy shift : Moscow will shoot down any U.S. missiles fired at Syria (not just the missiles fired at Russian soldiers) and it will target the launch sites . The escalated threat may have been unintentionally improvised, per The Guardian."

: Moscow will shoot down fired at Syria (not just the missiles fired at Russian soldiers) and it will target the . The escalated threat may have been unintentionally improvised, per The Guardian." "How is Zasypkin’s rhetoric different? On March 13, Valery Gerasimov, the head of Russia’s General Staff, said, 'Russia’s armed forces will take retaliatory measures against the missiles and launchers used” in attacks that pose “a threat to the lives of our servicemen.'”

Trump's new tone

As Axios' Jonathan Swan notes, Trump still believes the U.S. and Russia have plenty of shared interests and has long believed it would be important to have a warm personal relationship with Vladimir Putin.

Even as he declares a new low in relations, Trump is trying to keep that possibility alive: "There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together," he writes.

Last month, Trump invited Putin to meet to discuss the arms race. Russia has recently tested what it calls an "unstoppable" nuclear-capable ballistic missile.

Worth noting: As a candidate, Trump repeatedly mocked Barack Obama for telegraphing his moves in the Middle East.

Sign up for the Axios World newsletter to get more stories like this.