okay by security... what do you mean by that? as in why they were on guard from the US? if so, after the WW1 a bunch of conflicts and diplomatic issues arrived between the US and the soviet union, the start of making weapons and nuclear warheads and the thought of sending these deadly weapons across the planet was during this era, and the thought of that makes these two countries weary and suspicious of each other. after all the US abandoned the Soviet during the fight at czechozlovakia (i hope i spelled that right) . furthermore the US and the soviet union had different view points. Thus the Soviet union planted some of their nuclear warheads near the US since the US placed some in Turkey that could reach Stalin. so yeah major trust issues and past experience are all whats causing these which turned to the Cold War.