Civil wars are fought between citizens of the same country, and are fought within the country.
If citizens of a country disagree about a policy or leader, they may fight a civil war.
A world war is fought when countries form alliances, and these alliances fight each other.
A world war involves many or most of the world's most powerful countries.
There were 2 world wars in the 20th century.
Terrorism involves groups of individuals rather than countries.
Terrorists may come from different countries, and they may share the same beliefs.
Terrorists often use car bombings and suicide attacks on innocent people.
A terrorist attack can happen at any time and any place.
An invasion occurs when one country attacks another.
It sends its army into the other country, and attempts to take control.
One of the most famous invasions was when France invaded Russia in 1812.
A defensive war occurs when a country is attacked and needs to defend itself.
Most countries have a military, so that it can defend itself if it's attacked.
When France invaded Russia in 1812 the Russian army fought a defensive war and defeated the French army.
(Sometimes two leaders will fight for power within a country and start a civil war.)
(Throughout history there have been many invasions.)
(The influenza virus spreads through the air when people cough or sneeze.)