Former Attorney General Bill Barr said the United States cannot allow Russian President Vladimir Putin’s invasion of Ukraine "to escalate into a nuclear war," warning that now is the time "for prudence" and to avoid any "direct conflict" with Russia.

In an interview with Fox News Digital about his new memoir, "One Damn Thing After Another," in which he details long-term national security challenges facing the U.S., Barr said he agreed with the Biden administration's moves to reject Poland's proposal to send MiG-29 planes to the Ukrainian military. 

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The Pentagon this week said Poland's proposal raised concerns regarding the deployment of warplanes from a U.S. base in a NATO-allied nation to combat Russian forces.

Ukraine has been pleading with Western allies to send military aid and implement a no-fly zone over the country. The U.S. and its NATO allies have repeatedly said a no-fly zone over Ukraine could bring about a direct conflict with Russia.

Attorney General William Barr 

"We have to be very careful about getting into direct conflict with Russia — Russia is a nuclear power, and we can’t allow this to escalate into a nuclear war," Barr said. "I do think now is the time for prudence, and that we do not escalate — I know people see what’s going on, and they feel terrible about it — but that’s the nature of countries getting nuclear weapons: our options are curtailed."

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"That’s why having North Korea get nuclear weapons is a problem, and that’s why Biden’s efforts now in reopening these negotiations with Iran and letting Iran become a nuclear power is very sobering," Barr continued. "After Ukraine, say some things like this happen in Israel — we would be dealing with a nuclear power."

Putin launched Russia’s multi-front war against Ukraine on Feb. 24. His brutal invasion of the country is continuing for a 15th day, and Barr warned that there could be a long road ahead.

"I think now that Putin has committed, there is no off-ramp for him," Barr said. "It is hard to see a way he saves face without continuing to push forward."

Vladimir Putin

Russian President Vladimir Putin has signed into law legislation that could punish journalists with up to 15 years in prison for reporting so-called "fake" news about his military invasion of Ukraine.        (Yuri Kochetkov/Pool)

Meanwhile, Barr said he does not think Russia has "the military power" to move into neighboring countries after the invasion of Ukraine.

"Even if their military was operating well, I think they’ve diminished themselves in the eyes of the world as a military power, because their military is not functioning very well," Barr said. "But even on paper, they don’t have the military power to do more than what they’re doing."

Barr added that, over time, he believes Putin’s strategy will be to "neutralize" and have "a puppet regime in Ukraine like in Belarus."

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"And so, that Ukraine, Belarus, Russia would be his sphere of influence, and then, down the road, I think he could try to destabilize the Baltic States," Barr said.

"I don’t think there is any risk in the short term of him expanding this to include invasions of other countries," Barr continued. "I don’t think that there is a game plan to move beyond Ukraine at this point."

But Barr said Biden's weakness "invited" Putin's invasion of Ukraine. 

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"A lot of our foreign adversaries think President Biden is a weak leader, and I think that’s partly what invited the aggression in Ukraine," Barr said. "Putin has always made it clear that he would not tolerate Ukraine as a part of NATO, but there was no real urgency on that."

Joe Biden

Former President Trump long referred to his political rival as "Sleepy" Joe Biden.  (AP Photo/Andrew Harnik)

"I just think that he saw that there was an opportunity for him to move quickly against a leader who he didn’t think would resist very effectively," Barr explained, noting that the opening was seen during the botched withdrawal of U.S. military assets from Afghanistan.

"Putin also saw that Biden had curtailed American energy independence, thus increasing the leverage of Russia," Barr said.

"He didn’t do anything by moving equipment and resources into Ukraine before hostilities began, which might have deterred Russia," Barr said. "If he had put planes there and built up the Javelin missiles upfront, you may have had more of an effect in deterring it."