Question

What are two examples of a just war

Answer 1

Two examples of a just war are when a) poland were at war with tthe teutonic knights (also known as the knight sof the cross), and when b) the kingdom of spain wanted to take over the americas when they were first discovered. 

Answer 2

War in defence of one's country is almost always just, since it's not your decision to wage war, you are just acting in self defence. 
Examples:
-Chinese - Japanses war in 1937, was from Chnese perspective just, as they were defending their territory.
-Polish - German war in 1939 was from Polish perspective just, as they were defending their territory.
In both cases aggressors had no valid/just claim for aggression, other than expansionistic policy,

As for "preventative wars":
In just war theory war is  just when:
-a valid reason to go to war exists (war may be cosidered lesser evil).
-war itself must be conducted in civilized manner.
Since i think every moral codex justifies killing someone, when that person is direct and immediate threat to life of somebody else, you may extend this rule on a larger scale.It's hard to find such examples in history, even if there war was waged with valid reason (genocides), it usually was not conducted in civilized manner (because its not efficient).