How are the armies aligned?

The Achaeans (Greeks) tried to acquire Helen by diplomatic means, but when they were unsuccessful they gathered in Aulius.

The Achean forces first gathered at Aulis. All the suitors sent their forces except King Cinyras of Cyprus. Though he sent breastplates to Agamemnon and promised to send 50 ships, he sent only one real ship, led by the son of Mygdalion, and 49 ships made of clay. Idomeneus was willing to lead the Cretan contingent in Mycenae's war against Troy, but only as a co-commander, which he was granted. The last commander to arrive was Achilles, who was then 15 years old.

The opposition (Troy) occupied the city of interest to the Greeks, so they were already formed, in that sense.