Countries with 2nd strike capability, will always be able to retaliate if they are nuked by someone else, making the action on of the attacking country a suicide, and as a result, will never occur in reality.
Nuclear weapons can always be used as a last resort if it seems like said country is about to lose the war and be destroyed in a conventional war. which means the attacking country would be committing suicide in an attempt to destroy said country, which means it will never happen in reality.
if the country has 2nd strike capability, the power to deploy nukes is in the hands of few and their motivation to win ignores the value of the country itself over retaliation the attacking country will be destroyed.
It all depends on the country. If we are talking about a small (area) country such as the ones on the European Peninsula or Africa. Then they can be defeated but nuclear weapons may be sent before the other weapons hit. But If it is a large country then it would be a lot harder to defeat it.