So imagine you are POTUS. You have just received a report that an alien spacecraft has passed by the ISS. It sent a single message before disappearing into the blackness: “They will be here within a week. Stand fast. Help will come.” The alien spacecraft was clearly damaged. The crew of the ISS took photos and have a recording of the message. The Russians have received the same data.
What do you do?
Lock and load. If ET shows up wanting a fight, give him one. If he shows up friendly, remain vigilant.
It could be that the first bunch are bad guys. Could be that they both are bad guys. Could be that they are both decent sorts that hate each other but are willing to leave us out of it.
Buy Haliburton stock. Lots of it.
They’re going to help us but their spaceship is damaged?
Get on the hot phone to Russia and say,
“What the heck we can’t even trust you guys”
“How can we trust a message from an alien space craft”?
“For all we know you guys are playing a trick on us”
or just pass out the cool aide these aliens will never let us live
Doesn’t mean they don’t have friends.
As to the OP… prepare to hide.
We can assume that if somebody has FTL drives, we’re going to lose any fights we get into with them.
Our only chance would be if they unwelcome guests were dumb enough to engage with us while lacking adequate fuel or ammunition to engage us.
One imagines that in such a visitor would choose to NOT engage us, so our only reason to engage and win is the chance that we’d be attacked by a mentally ill party, which I feel isn’t likely.
Harry Harrison wrote a (typically lumpy) take on this in one novel. Invasion: Earth.
Two alien races show up and try to convince Earth that the other is bad. It turns out they are working together to deceive Earth into doing something I forget…
Pick a different week to stop smoking.
Precisely.
Why? Sure if they have FTL they can disengage at will, but if we do get them to disengage, isn’t that the same as winning?
Go to DEFCON 1. Hold round the clock meetings with the Joint Chiefs of Staff, convene the United Nations Security Council. Order the National Guard mobilised. Co-ordinate with Federal and State law enforcement authorities to lessen damage from panic. Revive the old Cold War plans for continuity of government.
We come in peace, shoot to kill! Hope they aren’t religious, too. I’d also be extremely suspicious of a ploy. Assuming hostility is the safest thing to do and the best service to the nation.
Btw, in what format was the message received? Analyse the crap out of it to see if it’s faked, since it could also be a terrestrial-based hoax.
Semaphore. 
How do we know that they didn’t just mean to send “NUJV”?
But some of MY friends are Vulcans!
Be the first to welcome our new alien overlords.
Dust off the nukes.
Haven’t you seen Independence Day? All we need to do is get ready to upload a virus. :dubious:
William Tenn wrote a particularly bleak take on this scenario in “The Liberation of Earth”, with the two sides repeatedly “liberating” Earth from each other until the place is no longer worth bothering with. Sucks to be one of the natives…
Because if a civilization is so advanced as to be able to break the laws of physics at whim, they can destroy us at whim.
Might as well be native Americans back in the middle ages trying to hold of an invading army of modern carriers with a full compliment of F-16’s, helicopter gunships and nuclear weapons.
Basically, in this scenario, we better hope our “friends” can bring help quick, 'cause any civilization capable of waging war across interstellar distances is WAY beyond us. We have trouble waging war across time zones.
On the assumption that we’re sure its not a terrestial hoax all we as a species really could do is go to battle-stations as much as one week will allow and see what actions the second set of aliens take, if they’re hostile then we’d have to fight them as far as we are able.
Its actually a hopeful message:
(a) it indicates that the first set of aliens believes we have a fighting chance to hold the hostiles off at least for some time
and
(b) help is on its way
Interesting OP!
Based on my viewing of the factually-based movie “Battleship”, we should surround our planet with flimsy communications satelites and wait for the enemy fleet to pummel itself to bits on entry into Earth orbit.
Oh wait, we’re ahead of the game already. 