Deterrent, not deterrant. Also, Yes, countries without nukes are more vulnerable to attacks. Just imagine what would have happened had Saddam Hussein got nukes before he invaded Kuwait. In such a scenario, the West might have been much more hesitant to militarily intervene.
Its like being one of few tough guys who primarily use fisticuffs but getting a gun. You're a badass until you get into it with someone with their own gun, especially someone with more guns, better guns, who is highly skilled with them. Suddenly the idea of being a gun fighter doesn't sound so hot.