If you've spent even five minutes listening to the news about Russia's ongoing military invasion of Ukraine, you have noticed something strange: Western leaders, from President Obama to the head of NATO, are bending over backwards to call the invasion anything other than "invasion." Popular substitutes have included "aggression," "escalation," and especially "incursion."
The rhetorical backflips have gotten so silly that outraged Ukrainians started a hashtag campaign, #RussiaInvadedUkraine, to try to force the world to acknowledge the invasion that everybody knows is a huge terrible problem but no one wants to admit is happening.
So why are Western leaders making this transparent and awkward effort to not call the invasion an invasion? There are three plausible factors at play here, but the third is the main one: they want to leave Russian President Vladimir Putin a face-saving way to withdraw from the conflict. None of these is a hugely satisfying answer, but keep in mind that the US and Europe don't actually get anything for calling it an invasion, so they may see the tradeoff as minimal.
1) Obama and NATO want to leave themselves space to escalate rhetorically
Russia's invasion of Ukraine is definitely an invasion, but it is not a full-scale invasion. The Russian military is one of the largest in the world with about 845,000 active-duty troops and thousands of tanks and combat aircraft.
So far, Russia has sent in a few tanks, some convoys of self-propelled artillery, and perhaps 1,000 troops — an invasion, yes, but just a small fraction of what Russia could send, and not enough to overrun the Ukrainian military.
Western leaders, then, may be holding the word "invasion" in reserve in case Russia does launch the sort of full-scale invasion it is capable of, with the intent of matching the military escalation with a rhetorical escalation.
2) The US and NATO don't want to highlight how serious this is, since they're not intervening against it
The US and NATO are definitely, definitely, definitely not going to send in troops to defend Ukraine against Russia's invasion. Ukraine is not a member of NATO, so it has no mutual self-defense treaty with Western countries.
Sending American or European troops into Ukraine would mean it is very likely that they would end up fighting either Russia-backed separatist rebels or Russian troops, something that the US and Soviet Union avoided for virtually the entire Cold War. Direct fighting would make it very likely that the conflict in eastern Ukraine would escalate out of control to full-on war between nuclear-armed powers.
President Obama and UK Prime Minister David Cameron care about Russia invading Luhansk and Donetsk, but they're not going to risk global thermonuclear annihilation over it. The theory, then, is that Obama and Cameron and others don't want to call it in an invasion because that would highlight their refusal to intervene against this very bad thing that's happening. I'm not sure I buy this theory, to be honest — anyone in the US or UK voting publics who would care about this already knows it's a Russian invasion — but it is one you hear.
3) The West wants to leave Putin with a face-saving way to back down
This is the one that seems the most plausible: by going along with the idea that it is not a Russian invasion, that makes it easier for Putin to pull back troops at some point and pretend that he never invaded in the first place.
Putin insists that Russian troops are not invading Ukraine right now. The US and Europe are not really going along with this fiction; they are very clearly pointing out the Russian troops and tanks flowing over the border. But they are sort of buying into the idea that Russia is not outright invading Ukraine.
This avoids a rhetorical escalation — the West does not want to escalate, it wants to de-escalate — and the likely hope is that it makes it easier for Putin to quietly agree to a peace deal by which he pulls back troops. If he never has to acknowledge that he invaded, then it will be less embarrassing for him at home to end that invasion, particularly in defeat. The US and Europe know that Putin is invading in large part for complex domestic political reasons, so while being sensitive to Putin's domestic political needs is obviously very distasteful, it may make it easier to negotiate a peace deal.