Invisible attacks by states like Russia, China and Iran risk unravelling the world's democracies "without a shot being fired", the defence secretary and the UK's military chief have warned.
There is also a threat that hostilities - from cyber hacks and disinformation to assassinations and sabotage - escalate, perhaps even triggering an "uncontrollable state of war", they said.
A new podcast series by Sky News explores how assaults in a grey zone between war and peace can weaken a target society from within by eroding trust in government and amplifying divisions on issues like politics, race, and even the coronavirus pandemic.
They take place deliberately below the threshold of war so as not to draw enough public attention to prompt an effective retaliation, according to interviews with current and former spies, hackers and soldiers for the Into The Grey Zone podcast.
The whole point is to cause harm to your enemy and gain an advantage without people feeling sufficiently threatened or realising they're under attack, they said. Left unchallenged, these "insidious, subversive" attacks could be just as damaging as a military invasion.
Lieutenant General Graeme Lamb, a former director of UK special forces, summed up the threat: "We're being boiled like a frog."
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