berlin nato defend sweden finland event arm aggression although neither country alliance member senior nato official say wednesday event underscore grow concern russia military buildup commodore hans helseth special adviser nato joint warfare center norway say two nations grow tie western alliance increase risk nato moral obligation come assistance attack non-membership actually low threshold arm aggression two countries opinion nato simply moral reason reason come two countries assistance helseth say berlin security conference say nato remain open welcome nations members choose join finland sweden denmark norway iceland agree month step defense cooperation exchange air surveillance information part broad effort build defenses counter russian activities officials finland sweden speak conference concern russia military buildup arctic northern europe general cite 2014 annexation crimea region ukraine wakeup call two countries take part nato military drill cover article nato foundational treaty say attack one member attack countries know move join alliance would create backlash moscow oppose expansion nato senior russian foreign ministry official tuesday call efforts rebuild tie european countries include communication among military experts helseth say could return business usual russia give annexation crimea role support violence eastern ukraine well support syrian president bashar al-assad use unfriendly attack cyber domain top military leaders nato member countries speak two-day event efforts build big deterrent military aggression russia u.s. lieutenant general ben hodges head u.s. army force europe say russia miss big opportunity mend tie west transparent military exercise year u.s. nato officials say russian force far exceed 13,000 threshold beyond country obligate invite military observers russia say western officials exaggerate scope exercise hodges say western nations achieve unprecedented level information-sharing exercise say hop would continue even nato expand capabilities rapidly move force eastern flank cite concern recent activities russia include purchase 2,000 vehicles transport tank road move take advantage melt ice arctic russia sit around wait see happen nato defend sweden finland attack nato official