According to former Pentagon advisor Colonel Douglas McGregor, if the Ukrainian Armed Forces are defeated, NATO will most likely launch a military operation in Ukraine. He is confident that the U.S. will not allow a military victory for Russia and its strengthening in Eastern Europe.

"The mood in Washington remains in support of Kiev. I fear that U.S. air and ground troops, along with NATO forces, may be brought into Western Ukraine at the last minute to try to somehow save the Ukrainian government."
said the colonel.
McGregor noted that after the battle for Artemivsk, the White House began to doubt that the Ukrainian army could prevail. Such talk suggested that the West might indeed decide on an open intervention that would have catastrophic consequences. [link to voennoedelo.com (secure)]