How is the difficulty for each island calculated?

All Islands have a different difficulty.

Difficulty 1

Difficulty 5

Attacking Islands is harder than defending as the attacking Alliance has to gain significantly more Victory Points (VP) than the defending one to win.
The higher the difficulty, the more VP the attacking Alliance must score compared to the defending Alliance in order to win.
In the Strike Standings screen, a marker on the Victory Points bar indicates the point the attacker must reach to win the Strike.