(AGI) Trapani, Oct 19 - NATO's biggest exercise since 2002starts on Wednesday. In the scenario, which runs until Nov. 6,one country invades a smaller neighbour and then threatens toattack another. The UN Security Council authorises NATO tointervene in defence of the invaded country and to prevent thethreatened attack. NATO begins operation "Trident Juncture2015" with 36,000 soldiers, totalling 230 units, from 28 NATOcountries and Finland, Sweden, Austria, Bosnia and Herzegovina,Macedonia and Australia. Also taking part is Ukraine which foryears has been interested in joining the Atlantic Alliance. Theoperation also mobilises 140 aircraft and 60 ships. The theatreof operations is Italy, Portugal, Spain, the Atlantic Ocean andthe Mediterranean. Observer countries include Brazil, Mexico,and Colombia. There are also NGOs and humanitarianorganisations, and there will also be international defencecompanies for the first time. The exercise aims to developflexible responses to Islamic terrorism, mainly ISIS. The nameof the invading country, purely theoretical, is not revealed.Four thousand Italian soldiers will be taking part. The openingceremony was held on Monday at the airstrip of the Italian AirForce 37th Wing in Trapani Birgi, in Sicily, which will be oneof the Italian venues involved, with Cape Teulada (Sardinia)for the army, the central Mediterranean for the navy and airsupport from Grosseto, Pisa, Amendola (Foggia), Decimomannu(Sardinia). The national flags of the countries in theexercise and the NATO flag were presented at the openingceremony.. .