THE PROSPECT that the Trump administration will soon develop friendly relations with Russia appears to be fading fast. Though President Trump has yet to make a critical remark in public about Vladi­mir Putin, his aides are lambasting the Kremlin for its tolerance of, and possible complicity in, a chemical-weapons attack by Syria. White House aides are even denouncing the “fake news” reports with which Moscow is attempting to sow confusion about the use of the nerve agent sarin by the regime of Bashar al-Assad — which is certainly a change from Mr. Trump’s campaign habit of echoing Russian disinformation.

If the administration is developing a more realistic view of Mr. Putin and the threats he poses to core U.S. interests, that is to be welcomed. That said, there seems to be more learning to do. On his way to Moscow for meetings Tuesday, Secretary of State Rex Tillerson coupled his criticism of Russia, and a welcome declaration that “we see no further role for the Assad regime longer term,” with an expression of “hope” that Russia would choose to drop its alliance with Damascus and Iran and instead “realign with the United States, with other Western countries” that “are seeking to resolve the Syrian crisis.”

Mr. Tillerson might have been reading a script from his predecessor, John F. Kerry. For years Mr. Kerry doggedly repeated the phrase that Russia needed to make a choice in Syria between the Assad regime and those seeking peace — and over and over again, Mr. Putin chose Damascus. That will not change because a new administration has taken over in Washington. As Philip Gordon, a former senior official in the Obama administration, explained in a commentary for The Post, Mr. Putin is dead set against “regime change” in Syria or anywhere else the West seeks the removal of a dictator. Moreover, Russia’s new place of power in the Mideast, including its Syrian bases, is defended by Iranian-led ground forces, making a rupture of the Moscow-Tehran alliance unthinkable for Mr. Putin.

If Mr. Tillerson really wants Russia to switch sides, he will need to obtain what Mr. Kerry sought in vain — leverage provided by U.S. military action to cripple the Assad regime. One volley of cruise missiles won’t do it: The United States would need to destroy the rest of the Syrian air force and provide greater support to non-extremist rebels on the ground. That would be far harder to do now than in 2012 and 2013, when Mr. Obama chose not to act; among other things, it would risk a direct military conflict with Russia.

A more modest goal for U.S. diplomacy would be to agree with Russia on a de facto partition of Syria into zones controlled by the regime, Western-backed rebels and Kurds, with a long-term cease-fire imposed on all sides. Russia could meanwhile round up and dispose of the chemical stocks that the Assad regime still retains. That would, at least, spare Syrian civilians from further atrocities and allow for a concentration of military efforts against the Islamic State and al-Qaeda. But as Mr. Kerry could tell Mr. Tillerson, even that won’t fly with Mr. Putin unless the United States is willing to show greater resolve.