Learn English – What type of phrase is “I’m telling you,” when used for emphasis

You can divide this sentence into two separate clauses.

The dependent clause, which provides the reason, is:

To understand the importance of this event

while the independent (statement) clause is:

you should know all the facts

Identifying these two separate clauses, it is then clear that a comma should be introduced between them, by standard rules.

Hence it's most correct to write:

To understand the importance of this event, you should know all the facts.

This isn't actually a compound sentence, it's a simple sentence that contains a compound predicate. A compound sentence would look something like Dad caught fish, and he cooked them for supper.

So, the 'Naming Part' (subject) is simply the word Dad and the 'Telling Part' (predicate) would be caught fish and cooked them for supper.