AILSA CHANG, HOST:

After weeks of this lingering crisis, yesterday, the language about Putin and Russian aggression towards Ukraine seemed to have changed.

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JOE BIDEN: This is the beginning of a Russian invasion of Ukraine, as he indicated and asked permission to be able to do from his Duma.

ANTONY BLINKEN: Now that we see the invasion is beginning and Russia has made clear its wholesale rejection of diplomacy...

JENS STOLTENBERG: What we see now is that a country which is already invaded is suffering further invasion.

CHANG: That was President Biden, Secretary of State Antony Blinken and NATO Secretary General Jens Stoltenberg speaking yesterday about Russia's decision to order troops into parts of eastern Ukraine. So it might look like an invasion has started, according to some world leaders. But when it comes to a formal definition, are we there yet?

AYESHA RASCOE, HOST:

We asked Mary Ellen O'Connell, professor of international dispute studies at Notre Dame Law School and the Kroc Institute for International Peace Studies, how do we know when we've passed the red line?

MARY ELLEN O'CONNELL: An invasion is the crossing of an international border with military force. It can be a full-scale invasion that aims at taking over the entire country. Lesser invasions, we tend to use the term incursion.

RASCOE: And where on that scale do Russia's current moves fall?

O'CONNELL: So far, I would label Russia's military actions against Ukraine as of 2022 as incursions.

CHANG: OK, incursions. So why is it so hard to know for sure when it comes to Russia and Ukraine in particular?

O'CONNELL: Part of the complication is that Russia has already invaded Ukraine. In 2014, when they took occupation of Crimea, that was an invasion.

CHANG: And although Russia has always denied it, its forces have also been in eastern Ukraine, so Russia has already violated the border.

RASCOE: But O'Connell says the Russian military could still go further.

O'CONNELL: In international law, we would be looking for the Russian military to move all the way to Kyiv and to actually take control of the government and take control of territory in doing so. That's the maximalist act of aggression.

RASCOE: Meanwhile, the U.S. and its partners are trying to make sure that never happens.

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