1978 A.D.: Norse-German scientists make significant leaps in the field of internal combustion engines, putting mass-produced automobiles on the roads for the first time.
And, perhaps more importantly, letting me build tanks. It seems pretty late in the game to make this happen, but it's just the way this world developed. Everyone kinda forgot to invent tanks until the late 70s, despite having very advanced aircraft and naval units for the time.
1979 A.D.: Greece becomes the fourth nation to develop a manned space program. While everyone is occupied not caring about this announcement, the Celts and the English jointly denounce Rome.
So, this is interesting. Last I heard, the Celts and England hated one another. And England really has nothing to gain from attacking Rome right now: Russia has swallowed up all the Roman territory that used to border it. The Celts are still my longest-standing, most loyal allies, but I really don't have any intention of getting roped into an overseas war with Rome unless it's close to beating me to a science victory.
Rome goes to war with Russia again, looking to take back its Northern cities.
1981 A.D.: Norse spies report that the Greeks have nuclear weapons. They make some vague threats about the Norse military being weak or something.
While certainly a game-changer, I still basically control the geopolitical climate. Thanks to the standing defensive pacts I set up long ago with basically everybody, Greece would commit itself to another war against the rest of the world by attacking me. Any further wars between us will be on my terms, and I'll have the initiative. Using said initiative, I'll make sure to cripple its nuclear stockpiles before they get dumped on me.
I've also just researched Combined Arms, giving me access to anti-tank guns. Greece has tanks now, making this an opportune development.