@fansy,
The US government has abandoned a policy it had for a long time.
The policy it used to have was that it would not threaten to attack countries that did not have nuclear weapons unless they attacked first.
"First strike" means that one attacks before the other country does...usually when you believe that the other country is about to attack you.
The US used to say that it would not strike first against a country that did not have nuclear weapons.
Now it says that it will attack anyone before they attack the US.