'A few months ago," tweeted Foreign Minister John Baird on Thursday, Prime Minister Stephen Harper "described his fears of a slow-motion invasion in Ukraine. It appears Russia is starting to turn up the speed." Mr. Baird is right.

First, Russia sent arms and thinly disguised insurgents across the border. Then it sent heavier weapons, such as the surface-to-air missile system that shot down Malaysia Airlines Flight 17. Then Russian artillery fired from across the border, in support of Moscow's proxies. Then, according to a NATO report last week, Russian artillery itself crossed the border into Ukraine. And on Thursday, Russian troops crossed the border and seized the southern Ukrainian town of Novoazovsk, opening up a new front in the fighting. NATO says that, as of Thursday morning, 1,000 Russian troops were operating in Ukraine.

We should name this for what it is: an invasion. And not just that. It is also an attempt to change the long-recognized borders of a sovereign state, something Moscow did earlier this year when it carved off Crimea. All of it is illegal under international law, and unprecedented in postwar European history.

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Moscow's response? Denial. Lie and lie big. "We have noticed the launch of this informational 'canard' and are obliged to disappoint its overseas authors and their few apologists in Russia," defence ministry official Major-General Igor Konashenkov told Interfax news agency in response to evidence the Russian army is in Ukraine. "The information contained in this material bears no relation to reality."

This is precisely how the script was played last time, too. Earlier this year, Russian President Vladimir Putin spent weeks denying that his troops had occupied Crimea – then flew to the peninsula to take part in a huge military display celebrating the successful occupation of Crimea. Moscow says the peninsula is now part of Russia.

The West must respond. It should immediately slap stronger economic sanctions on Russia. It should send greater economic aid to the government of Ukraine. And at the NATO meeting next week, it should consider sending military assistance, too. The Western Alliance is not going to go to war with Moscow over Ukraine, and Mr. Putin knows this. NATO troops will not defend Ukraine. But we can at least challenge him on the ground by putting more and better weapons in Ukrainian hands. And we can kneecap his economy with sanctions.