If the Roman Senate's law says the army can't cross the Rubicon river, and anyone who does lead an army across will be executed - then that's the law.

But if an army actually crosses the Rubicon? Turns out that law doesn't count for much when the other guy has an army, and all the senators have to flee the country.

Then the guy with the army is in charge. He can retrospectively legalise his actions, grant himself a pardon, and fill recently-vacated positions of power with allies who'll sign it all off.

So while the exact details of "Title 10" and "Posse Comitatus" are very important to an army before they cross the Rubicon - if they do cross the Rubicon the law will suddenly turn out to be far more flexible than anyone expected, and it'll turn out their actions were legal after all.