While Russia's army continues missile and drone attacks on civilian targets in Ukraine, there is a debate in Germany about whether Ukraine's military should be striking back across the border.

The debate was triggered after Ukrainian President Volodymyr Zelenskyy said the war was "gradually returning to the territory of Russia — to its symbolic centers and military bases, and this is an inevitable, natural and absolutely fair process."

On Monday night, Russian authorities said its air defenses had shot down a drone near Moscow and two others in the Bryansk region on the border with Ukraine. There were no reports of casualties or damage.

Russia says its air defenses had already repelled similar drone attacks over the capital on August 23 and July 4.

'Deter, prevent, repel'

Under international law, countries may defend themselves if they are attacked. "Under those circumstances — cross-border aggression — Ukraine does have the right of self-defense," said David Scheffer, who teaches law at Arizona State University. "And that right of self-defense includes military strikes on Russian territory in order to seek to deter, prevent, repel the acts of aggression against Ukraine." Such "territory" is not limited to the front line.

International law does not permit attacks on civilian populations. "It is a legitimate act of self-defense for Ukraine to actually strike very deep into Russia, including into Moscow, as long as those strikes hit military targets," Scheffer said. "If any of those drone strikes hit civilian targets in Moscow, then that would be illegal unless it could be somehow proven someday in a court of law."

It has not been confirmed who is sending the drones and which targets they are intended to strike.

International law permits attacks on territories occupied by Russia, such as the Crimean Peninsula, which legally belongs to Ukraine, and on Russian naval targets in the Black Sea, but not on civilian merchant ships.

Limited weapons selection

In principle, the use of all applicable weapons, drones, missiles, aircraft, and cruise missiles is permitted. However, Ukraine has made political agreements with its allies to not use Western-made weapons to strike Russia's hinterland.

NATO members fear that Russia could use the deployment of Western-made weapons as a pretext to escalate the war. In May, the US military's chairman of the Joint Chiefs of Staff, General Mark Milley, said that there was a clear understanding that weapons from the United States should not be used on Russian territory.

"This is a Ukrainian war," he said. "It is not a war between the United States and Russia. It's not a war between NATO and Russia. This is a war between Ukraine and Russia, and we are supporting and supplying and help training and advising and assisting Ukraine. But it is not a direct conflict between the United States and Russia."

Defining 'red line'

The red line is different for other governments. Britain and France are willing to deliver long-range cruise missiles that could allow Ukraine to hit troops and supplies behind the front lines.

"I have decided to increase deliveries of weapons and equipment to enable the Ukrainians to have the capacity to strike deeply," French President Emmanuel Macron said at NATO's summit in July.

The range of the artillery that Ukraine has been receiving from Western countries has gradually increased. Long-range artillery is needed to strike positions and airfields, from which the Russian army is launching missiles.

Ukrainian Ambassador to Germany Oleksii Makeiev has asked Germany to also supply cruise missiles. Though Agnieszka Brugger, the Greens defense spokesperson and parliamentary group leader, did not seem to be averse to the idea when she appeared recently on the German broadcaster Deutschlandfunk, Defense Minister Boris Pistorius, a Social Democrat, has so far rejected the request.

Party to war?

The moment when a third country becomes a party to war is not entirely clear in international law. Many experts think that the supply of weapons and munitions to Ukraine does not constitute active participation on the parts of the government's international allies in its war against Russia.

The question of whether training Ukrainian soldiers to use certain weapons constitutes active participation is disputed. Many legal experts believe that, as soon as states supplying arms have a direct influence on how long-range weapons are used and which targets in Russia are selected, they begin to actively exercise the "collective right to self-defense" with Ukraine.

The same would apply in the event that the United States, Poland or another country were to send its own troops into the combat zone. If any Western air force intervened, this would also be considered "collective self-defense," and thus participation in the war.

Russia's 'enemy states'

The Kremlin considers many NATO countries that are supplying weapons to Ukraine to be belligerents. "These are enemy states because what they are doing is war," Kremlin spokesperson Dmitry Peskov said in 2022.

Russian politicians have repeatedly warned that the country might be forced to use nuclear weapons to defend itself. The Kremlin could escalate its war and attack countries that support Ukraine and supply it with weapons. This would be an extension of Russia's war of aggression on Ukraine, which has been condemned by the UN General Assembly.

The European Union's spokesperson for foreign affairs, Nabila Massrali, told DW that it was not known whether the drones shot down near Moscow were from Ukraine. She said the Kremlin's statements were not credible.

"Russia shall not use this as another pretext for a further escalation of its illegal and barbaric attacks against Ukraine and its people," she said in Brussels, adding that it was clear that Ukraine had every right to defend itself. She said it had been agreed that weapons funded by the European Commission should be used solely for self-defense.

This article was originally written in German. It was first published on August 3, 2023 and updated on August 28, 2023 to include information about the latest drone attacks on Russia.