Short of invoking the Article Five mutual defence clause -- triggered only once before -- the answer is a mix of political messaging and gradually-escalating responses calculated to project the Atlantic Alliance's deterrent might.

The best-known of NATO's response options, Article Five states that an armed attack against one member shall be considered an attack against all.

It "underpins all of NATO's broader activities in the field of deterrence and defence, including the regular conduct of military exercises and the deployment of NATO's standing military forces," the alliance says.

But Article Five has been invoked only once in NATO's 77-year history, following the September 11, 2001 attacks in the United States. There has been no suggestion, so far, of doing so after the drone hit a residential building in Romania.

The latest example of the Ukraine war spilling over into the European Union nevertheless prompted a carefully designed response.

To start with, NATO swiftly expressed solidarity with one of its 32 member states, first on X via its spokeswoman and then through Secretary General Mark Rutte who personally reached out to President Nicusor Dan.

It pinned the blame squarely on Russia's "reckless behaviour" while pledging to protect alliance territory and strengthen defence capabilities on its eastern flank.

As a second stage, and only if the country concerned requests it, allies could meet for emergency "consultations" under the alliance's Article Four.

Calling mere talks is a step well short of triggering Article Five -- and may not seem very significant -- but NATO says the move has real impact.

In diplomacy, every word counts, and Russia can clearly distinguish between each level of reaction, according to officials in Brussels who say triggering Article Four is well understood as an additional step -- beyond statements and condemnations.

Article Four states that allies "will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the parties is threatened."

This is something the Russians take note of and which, in the past, has been able to modify their behaviour, officials in Brussels say.

- Article Four precedents -

Consultations under Article Four have been triggered nine times in NATO's history, including three times during Russia's full-scale war on Ukraine.

The article was invoked just after the invasion in 2022, and again twice in September 2025 -- by Poland after incursions by Russian drones, and by Estonia after Russian fighter jets violated its airspace.

The closed-door consultations often lead to decisions which -- although not always highly visible -- are aimed at strengthening the alliance's deterrence capacity, a central pillar of the organisation founded in 1949.

The Article Four meeting requested by Poland in September 2025 led to the launch of the "Eastern Sentry" operation to reinforce NATO's eastern flank.

The operation saw allies contribute "additional capabilities and assets to NATO's deterrence and defence," according to NATO's Supreme Headquarters Allied Powers Europe (SHAPE), including "more fighter jets, helicopters, transport aircraft, air defence systems, surveillance aircraft and frigates."

While it did not follow Article Four talks, the "Baltic Sentry" operation decided in January 2025 was also seen as a deterrent signal to Moscow -- after acts of sabotage attributed to Russia in the Baltic Sea.

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