with 3 yrs to act in a completely undetected manner engineer alone could force all of the earth's countries into an unconditional surrender by simply commandeering the entirety of earth's nuclear arsenal.... not sure if that would constitute a victory. or superman (assuming he is still undetectable) could spin to the center of the earth and destroy the core resulting in planetary destruction. not sure if THAT would be a win. if so, that'd be easy. being undetectable until they choose to be otherwise is a very very massive advantage. firestorm, gl, fate and probably even and mm and ww could destroy earth quickly were they totally undetectable.
if there is the need to actually conquer all the heroes and entities that reside on earth, that is something else altogether....
it would have to be pretty subtle to begin with. certainly not taking over with force. if the team worked together to quietly remove the heavy hitters i guess they could get it done. as soon as a couple 'disappeared' though, wouldn't that constitute 'making their presence known?' the rest of earth would definitely notice that and start becoming suspicious. that could set about a number of plans and the element of surprise would be gone--even though earth may not know exactly who it's up against, they would know something was happening and they would know it was a very serious threat.
depending on what defines victory here, i don't think this would be as easy as i've been reading.....
unless galan just turns everyone to salt. 😐