The tweet broke with what U.S. national security policy refers to as Operations Security.

"Get ready Russia, because they will be coming, nice and new and 'smart!'" Trump tweeted.

The tweet came on the heels of an alleged chemical weapons attack believed to be carried out by the government of Syrian President Bashar Assad. As chatter about possible U.S. retaliation grew, Russia said it would shoot down any U.S. missiles launched in Syria.

President Donald Trump 's tweet Wednesday morning threatening a potential U.S. strike against Syria broke with national security procedures — as well as his own admonishments about tipping off enemies about attack plans.

Secretary of Defense Jim Mattis and President Donald Trump are seen following a meeting at the Pentagon in Washington, D.C., July 20, 2017.

The OPSEC objective is to not publicly announce information that can be used by an adversary to jeopardize a military mission. The Department of Defense lists "social network sites, tweets, text messages, blogs, videos, photos, GPS mapping, newsletters" as a few of the ways in which OPSEC can be compromised.

"This is not a video game. The president of the United States needs to be calm and needs to act presidential," former U.S. Ambassador Nicholas Burns said in an interview on CNBC's "Squawk Box." "The tweet this morning was irresponsible. And it wasn't presidential. It wasn't effective."

The White House did not immediately respond to a CNBC request for comment on the potential military action.

"All I can provide about the President's statement is we do not comment on potential future military operations," a Department of Defense spokesperson told CNBC in an emailed statement. "The president has a wide variety of tools available at his disposal. I am not going to comment on or speculate as to what those options are or what decision the president might ultimately make."

Trump's tweet came amid indications that the U.S. could be preparing a military strike. Yet beyond the "get ready" warning, or its mention of "smart" missiles, the tweet lacked details about a possible strike.

"This tweet doesn't specify what or when or how many" missiles the U.S. would launch, Thomas Karako, director of the Missile Defense Project at the Center for Strategic and International Studies, told CNBC in response to Trump's tweet.

The White House said Tuesday that Trump "will not attend the 8th Summit of the Americas in Lima, Peru or travel to Bogota, Colombia as originally scheduled."

Instead, he will "remain in the United States to oversee the American response to Syria and to monitor developments around the world."

Likewise, Secretary of Defense James Mattis canceled weekend travel plans to Nevada and California, signaling that preparations for a military mission are underway.