I have not seen this phrase before and it comes from IISS in an introduction to the forthcoming publication 'Strategic Survey'.

It opens with:
The methods by which countries gain strategic advantage are often both innovative and brazen. The ways in which such state action can be deterred or countered are still in development. Uncertainty is heighted by the fact that some geopolitical moves have no obvious or immediate counter. ‘Tolerance warfare’, a style of geopolitical challenge that appears to be a preferred technique of the status quo disrupters, is becoming more prevalent. Tolerance warfare can be defined as the persistent effort to test the tolerances for different forms of aggression against settled states. It is the effort to push back lines of resistance, probe weaknesses, assert rights unilaterally, break rules, establish new facts on the ground, strip others of initiative and gain systematic tactical advantage over hesitant opponents. The purpose of tolerance warfare is to stress-test the ability of the target to deter and defeat these efforts, and then to win advantage either by diverting the target’s resources away from a central strategic purpose, or by creating new conditions that cannot be reversed except by expensive strategic effort that is perhaps disproportionate to the loss otherwise sustained. Sometimes tolerance warfare is conducted overtly and is in effect ‘declared’; often tolerance warfare is conducted through proxies or partners, especially in the most immediate theatres of operations.
Going local for a moment:
Indeed, an ancillary feature of tolerance warfare is the deliberate effort to create diversionary narratives that are aimed at obfuscating the facts and sowing confusion in international public opinion. All this will continue for the foreseeable future.....The year 2018 will be remembered as a turning point: new measures were found by Western states to drain power and capacity away from Putin and his circle, however difficult that might be.
Link:https://www.iiss.org/publications/st...8-005056be3f90