According to the NATO definition of hybrid warfare, “Hybrid threats are those posed by adversaries, with the ability to simultaneously employ conventional and non conventional means adaptively in pursuit of their objectives”. NATO has been particularly concerned about Russian activities, from the annexation of the Crimea onwards, but the term “appeared at least as early as 2005”.
The RAND Corporation has identified the Baltic countries as being vulnerable to hybrid warfare:
“Potential Russian hybrid aggression in the Baltics can be divided into three categories: nonviolent subversion, covert violent actions, and conventional warfare supported by subversion”.
Russia has already used all of these techniques in Eastern Ukraine, so this is not an idle threat. It would be very difficult for NATO to intervene in a situation where the population appears to be actively welcoming a Russian presence. In the case of Crimea, a referendum that indicated a desire to become part of Russia gave at least a veneer of political respectability to the annexation.
Russia’s strategy has avoided provoking direct Western military intervention, but it has still been costly: as reported by the BBC, Crimea annexation: Putin admits sanctions ‘damaging’. Hybrid warfare is visible to other countries, even though some aspects may be hard to prove.
 “The term ‘hybrid warfare’ appeared at least as early as 2005 and was subsequently used to describe the strategy used by the Hezbollah in the 2006 Lebanon War”, according to an article entitled Hybrid war – does it even exist? which was available in March 2017 from http://www.nato.int/docu/review/2015/Also-in-2015/hybrid-modern-future-warfare-russia-ukraine/EN/.
 A Rand Research article, entitled Hybrid Warfare in the Baltics: Threats and Potential Responses, analysed the possibilities. It was available in March 2017 at https://www.rand.org/pubs/research_reports/RR1577.html, and contains links to more detailed articles on the individual Baltic countries.
 The BBC published an article on the Ukraine crisis on 12 August 2016, subtitled What’s going on in Crimea? it was available in March 2017 at http://www.bbc.co.uk/news/world-europe-25182823. It accused Russia of infiltrating “little green men”, sparking protests, supplying military equipment and holding a referendum in Crimea to claim political legitimacy.