BRUSSELS – NATO is stepping up aerial surveillance in the Baltic Sea, while France, Germany and Sweden are bolstering Denmark’s air defenses ahead of two summits in Copenhagen this week over a series of troubling drone incidents near the country’s airports and military bases.

The number of serious airspace violations in Europe has spiked this month, including by Russian warplanes . But not all NATO allies agree on how to respond. Poland is ready to use lethal force. Others say that must only be a last resort.

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Regardless of who is to blame in Denmark, European leaders believe that Russia is testing NATO . Military planners in Moscow can observe how Western forces react, and countering intrusions by relatively cheap drones is a financial burden on the allies.

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