US President Donald Trump’s 50-day pause ahead of possible secondary sanctions on Russia gifts the Kremlin a window to exploit the incremental gains of recent weeks in Ukraine’s east.

If they succeed, Vladimir Putin would be able to claim victory rather than negotiating on Trump’s terms. “Putin will not negotiate as a loser,” one of his longtime associates tells TIME by phone from Moscow.

But his frustration with Putin has grown. Last week, the president said the United States was taking “a lot of bullshit” from Putin. Today, he authorized a significant shipment of U.S. defensive weapons to Ukraine via NATO and threatened Russia with new tariffs if the war does not end in 50 days.