The unprecedented Russian drone attacks on Poland are both a test and a warning. How Europe and NATO respond could be crucial to security on this continent.

The Russians are past masters at what's called "salami slicing". Tactics that use a series of smaller actions to produce a much bigger outcome that otherwise would have been far more provocative.

The Kremlin is probing the West with gradual but steady escalation. A British Council building and an EU installation are bombed in Kyiv ; a senior EU official's plane's GPS is jammed .

On their own each provocation produces nothing more than rhetoric from the West - but new lines are crossed and Russia is emboldened.

Ukraine war latest: NATO chief sends message to Putin

Image: Vladimir Putin has a history of testing the West.

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