An enemy vs A enemy

Some examples from the web:

• An enemy has entered the Vatican.
• An enemy has entered the Vatican.
• Any friend of Camelot is an enemy of mine.
• An enemy we don't yet, fully understand.
• An enemy of the Soviet people.
• An enemy with an arsenal of artifact weapons would not only be dangerous.
• An enemy that is way more advanced than us technologically.
• An enemy more than twice our number.
• An enemy yesterday can be a friend today.
• An enemy we have kept at bay for centuries has finally decided to strike back.
• An enemy had saved my life by the violent murder of one of my best friends.
• An enemy we could not buy our souls from with ransom:
• An enemy you'd happily see go to battle and fed their own men
• An enemy who knows everything about us... what we do and how we do it.
• An enemy, a very dangerous enemy, has placed a price on my head.
• The inhabitants of two worlds annihilated by an unknown enemy. An enemy that possesses the yellow power of fear.
• I said, "An enemy?"
• An enemy's enemy is a friend
• We have a more dangerous enemy. An enemy who is closer and more cruel.
• Youth today see traditional morals as an enemy.

Some examples from the web:

• The Cape is also a enemy of the law, and he too will be brought to justice.
• But I must remind you, again if any member of our team is infected, should be treated as a enemy and must be eliminated.
• How could I say, how he wanted her because Elena Rosenthal became a enemy of the people?
• All the world will be your enemy, Prince with a Thousand Enemies.
• Your résumé suggests you made a few enemies.
• A dead enemy is one less concern.
• Times of war call for a greater enemy.
• Every steamship owner is a potential enemy...
• We are fighting a powerful enemy in a war of ideology.
• Nothing like a new enemy or a new old enemy to get people behind you.
• Every workman, shopkeeper who strikes will be a recognisable enemy, a confessed offender, and they can pass to the offensive.
• All the world will be your enemy, Prince with a Thousand Enemies.
• You've just made a powerful enemy.
• I decided to give you guys a common enemy... me...
• The Garuda is a strong enemy, but his pride clouds his reason.
• So, guess who has a sworn enemy.
• They're the only way to prevail against a stronger enemy.
• Remember, we all have a common enemy.
• Meanwhile... Tesler sets his sight on a new enemy.
• I assume we share a common enemy.