New presidents are protected from war and do not need to worry about defense at all. Their countries are always in Secured mode and cannot be attacked by anyone. Secured mode will remain in force indefinately and can only be turned off if the country leader wants to turn it off.
A country in secured mode cannot be attacked and can never attack another country with a president. It is able however, to declare war and fight a country that does not have a president, conquer it and build an empire. Such war will not change the secured mode, it will remain in place.
A player who has more than one country in a single world (called an empire in the game) can have secured mode for one country. The other countries in the empire do not have secured mode and in general, can participate in wars. These countries must make sure they are defended and may be attacked by others.
It is possible to set "Temporary war protection" for each country and prevent war. There are some conditions attached to this function. It is intended for periods when the user is on vacation or not available. Temporary war protection can be extended indefinately by use of boosters.
The Fearless Blue world does not have secured mode. Fearless Blue is a war world. New players enjoy 2 weeks of war protection (42 game months) and can turn temporary war protection on if they are on vacation or not available for a short period. In general, Fearless Blue is the place where wars are more common.
Playing the full game requires country leaders to build an army.
The army is divided into a defensive force, an offensive force and a strategic force. The defensive force can deter others from launching an attack against your country. It can also cause severe damage to any attacking force and reduce the effects of such attacks.
If you intend to conquer other countries, you will need to setup a strong offensive army, possibly with a navy and sometimes even nuclear weapons. The war game can lead to a great empire but also to destruction and bankruptcy.