President Trump's biggest test at the G-20 meeting in Buenos Aires this weekend was supposed to be his tariff faceoff with China's Xi Jinping.

Instead, he confronts an in-your-face challenge from Vladimir Putin, the Russian leader, whom he never criticizes directly. Putin's open military move against Ukraine just before the G-20 – an attack on three Ukrainian ships in violation of laws of the sea – is a direct insult to the U.S. president. It is a signal the Russian leader thinks Trump is a patsy, willing to swallow Russian aggression and Putin's lies.

It's not clear whether the two presidents will meet. (Trump equivocated on Tuesday, but the Kremlin said on Wednesday that a meeting was still scheduled.) One thing is clear: Unless the president challenges Putin directly on the Ukraine attack, the world (including China) will take notice.  And Putin will know he has a green light for further aggression against Ukraine, the Baltics, and elsewhere.

"It is time for the United States to show leadership and impose costs for open Russian aggression," says the Atlantic Council's John Herbst, a former U.S. ambassador to Ukraine. "This is the first time that Russia used its military openly [in Europe], and we can't let it stand without risking further use of Russian military force."

The latest attack traces back to Russia's invasion and annexation of Crimea in 2014, and its later war on eastern Ukraine via local proxy militias helped by Russian troops. Putin denied there were Russian troops in Crimea – until his denials became visibly ridiculous. He still denies Russia's continued military involvement in eastern Ukraine (just as he denies Moscow's cyber meddling in U.S. elections).

Until now, Western sanctions seemed to have prevented overt Russian military attacks on Ukraine.  But this week's aggression at sea is a perfect example of how Russia is trying to wreck the Ukrainian economy and bring the Kiev government to heel.

Russian warships fired on and seized three Ukrainian military vessels that were traveling from the Black Sea through the Kerch Strait into the Sea of Azov, a triangular inland body of water bounded by Ukraine, the Crimean peninsula, and Russia.

A 2003 treaty between Moscow and Kiev gives both countries free access to the Sea of Azov and to the Kerch Strait, which links the Azov and  Black seas. But the Kremlin wants to totally control the strait, which runs alongside Crimea. The Russians also want to gain full control over the Sea of Azov, which would allow Moscow to control maritime shipping from Ukraine's two main industrial ports.

The fact that Moscow's actions violate international law on freedom of the seas doesn't seem to bother the Kremlin (Moscow denies it).  Beijing, which is challenging Washington and Tokyo over freedom of navigation in the South and East China Seas, must be watching with interest.

"The Russians are doing this to see how Ukraine and the West will respond," says Stanford University's Steven Pifer, another former U.S. ambassador to Ukraine. "If the response is weak, they will be encouraged to do more."

Not surprisingly, Trump's response has been weak.  Although the U.S. ambassador to the United Nations, Nikki Haley, slammed Russian aggression, Trump's first response was to blame both sides, saying, "We don't like what's happening either way."

"They were gloating in Moscow," says Pifer. "They see there is a big disconnect between Trump and Haley."

The president later expressed some second thoughts about "that [Russian] aggression" but segued immediately into a slam at Germany and NATO for not paying enough for their defense.

If Trump wanted to stand firm, he would spell out the cost of Russia's military aggression directly to Putin.  If the Russian leader denied all, Washington and European nations could increase their naval presence in the Black Sea, or ban port calls from Russian ships based there or in Sea of Azov ports.  New sanctions could be considered. Or Washington could bolster Ukraine's defenses by sending surface-to-ship missiles. (Trump deserves credit for green-lighting antitank missiles for Ukraine, but more is needed now.)

However, if past is prologue, there is little reason to expect a firm Trump stance with Putin – especially as the Mueller investigation heats up.  Who can forget the president's shameful rollover at the Helsinki summit, when he stood beside Putin and said he believed the Russian's denials of election meddling?

So responsibility for standing up to Putin may rest with the incoming Democratic majority in the House — who will no doubt find strong support from many in the GOP.

"What Russia is doing in Ukraine will be a major area where we focus," says Rep. Brendan Boyle (D., Pa.), a member of the House Foreign Relations Committee, "especially because this president has completely laid down in front of this Russian president."

You'd think a U.S. president – especially one who touts his own brilliance – would understand the risk of kowtowing to Putin. You'd think he would grasp the message this weakness sends to China.

Perhaps light will dawn as Trump flies to Argentina.  If not, when it comes to Russia, it's up to Congress to defend America's foreign-policy values and interests – and to clarify that Putin's aggression is not cost-free.