Changing conception of security in 21st century:
Security in the past:
Relied on territory and geography.
Arms race of conventional weapons.
Absence of non-state actors.
Importance of political security i.e sovereignty, territorial integrity over cultural or economic security.
Ideology more important than cultural and civilisation
Conception of security in 21st century:
Erosion of importance of territory because of WMD.
Nuclear arms race, and nuclear deterrence.
Importance of cultural and economic security.
Ascendancy of non-state actors
Clash of civilizations, with major security threat from other civilizations and not ideology.

Here you go. I hope this is good enough. Any improvements and mistakes will be appreciated.