There are two things happening between Ukraine and Russia right now. First, Russian and Ukrainian leaders are meeting in Belarus to negotiate a peace deal in the Ukrainian conflict that Russia insists it has nothing to do with. Second, Russian military forces are crossing the border into Ukraine in what is clearly a hostile invasion and act of war. That includes Russian artillery, Russian tanks, Russian-trained irregular forces, and even uniformed Russian soldiers who have admitted on camera that they are Russian military ordered to invade by their commanders.

But the first piece of that is obfuscating the second. Getting Russia to agree to a peace deal on Ukraine requires lots of careful diplomacy, and that means that no one wants to formally acknowledge Russia's invasion of Ukraine, even though Russia is definitely invading Ukraine right now. That's how you have even the more hawkish figures in the Obama administration making statements like this:

The new columns of Russian tanks and armor crossing into Ukraine indicates a Russian-directed counteroffensive may be underway. #escalation — Geoffrey Pyatt (@GeoffPyatt) August 26, 2014

Russia’s military incursions into Ukraine- artillery, air def systems, dozens of tanks & military personnel–represent significant escalation — Susan Rice (@AmbassadorRice) August 26, 2014

When is a Russian invasion of a neighboring country an incursion and an escalation, but not actually an invasion? When acknowledging that is in an invasion would complicate diplomacy to the point that ending the invasion could become much harder.

But there's more behind this confusion than just careful diplomacy. Russian President Vladimir Putin learned a crucial lesson from Syrian leader Bashar al-Assad last year, when the latter got away with using chemical weapons against his own people.

How to cross an American red line: slowly

That lesson is this: the Western world can set all the red lines it wants — don't use chemical weapons, don't invade sovereign countries — but if you cross that red line just a little bit at a time, inching across over weeks and months, rather than crossing it all at once, then Western publics and politicians will get red-line fatigue and lose interest by the time you're across.

Had Assad just openly dropped tons of sarin gas on his own people one day, the world probably would have responded much more forcefully than it did. Instead, cynically but smartly, Assad spent months using very small amounts of chemical weapons at a time. He created lots of uncertainty around individual incidents: had the chemical weapons killed anyone? Been used at all? Were they used accidentally, or even by rebels who'd stolen them? He kicked inspectors out, then kept them in; he cooperated, then didn't.

By the time Assad fired sarin-filled rockets at the neighborhood of Ghouta, murdering hundreds of civilians, including a number of children, the world had already acclimated to the idea that Assad was probably using chemical weapons. Western leaders could only muster so much outrage, and Western publics could only be so shocked, because everyone sorta, kinda knew that this was already happening and was going unpunished. There was no clear moment at which Assad had crossed the chemical weapons red line, so one day we woke up and realized he was miles on the wrong side of it, and it was too late to muster enough global outrage to push him back.

Putin's stealth invasion, right out in the open

That's what Putin did in eastern Ukraine this summer. First he began quietly supporting the separatist rebels there, then arming them. Then an awful lot of military-trained Russian nationals started slipping in, as Moscow had done before when invading and annexing Crimea in March.

In July, in the weeks before boneheaded rebels shot down a civilian airliner and killed 298 people, Russia began shipping them high-grade military technology, including surface-to-air missile systems. Shortly after the airliner shoot-down, Russian artillery in Russia began firing at Ukrainian military forces in Ukraine — an overt act of war. Then NATO said it caught Russia sending an "incursion" force across the border, which Ukraine claimed to destroy. Later, according to the rebel leader in Ukraine, 30 Russian tanks and up to 1,200 Russian-trained troops did manage to cross. By August 22nd, Russian artillery units were reportedly crossing the border into Ukraine, and a few days later, Ukraine captured "rebels" who admitted to being Russian soldiers sent under formal order.

Russia's meddling in eastern Ukraine became a stealth invasion, which has become an overt invasion. But it was all done just gradually enough, and with just enough uncertainty around each incremental escalation, that Russia has managed to invade a sovereign European country, in the year 2014, without sparking any larger war or the credible threat of any substantial response beyond sanctions.

That's not to argue that Western countries should have responded with more sanctions (even that took substantial cajoling from the US), but the point is that Putin shrewdly prevented it from happening by playing on the vulnerabilities of Western democracies: short attention spans, rapid war fatigue, and the often-unwise pull of popular opinion.