what is an example in hisory or current events in which people have their "mettle tested" ?

Answer 1

Iraq war.
Whatever political goes on, it is the people who suffer. And if they get back on their feet, their mettle certainly gets tested.

If you understand the exact meaning, you will be able to write more yourself. So read here:
Mettle is how strong they are mentally

1.courage and fortitude: a man of mettle.
2. disposition or temperament: a man of fine mettle.
—Idiom 3. on one's mettle, in the position of being incited to do one's best The loss of the first round put him on his mettle to win the match.

Answer 2

Any war. The bombing of Britain by the Nazis in WW2. The Battle of Stalingrad, same war.