Use a comma after a long prepositional
Shrase.
After a long civil war that devastated his
native Nigeria the main character in Civil
Peace rebuilds his life.

Use commas to set off internal clauses or
phrases that interrupt the flow of the
sentence.
Elie Wiesel recalling his suffering in the Nazi
concentration camps often spoke out against
similar oppression.

Prep phrases in the beginning of a sentence are separated from the independent clause by a comma.

after a long civil war that devastated his native nigeria (prep), the main character in civil peace rebuilds his life.

internal clauses will also be separated with commas when they do not flow logically with the rest of the independent clause.

elie wiesel (ind. clause), recalling his suffering in the nazi concentration camps (int. clause), often spoke out against similar oppression (continued ind. clause).