Does the Assault keyword trigger when a unit moves to an open (undefended) battlefield?

Ruling: No, Assault does not trigger when moving to an open battlefield. Assault requires a combat with an attacker and defender, but moving to an open battlefield is a showdown without an opposing unit, so there is no attacker/defender roles.

Nuances:

• This means units with Assault buffs can be targeted by removal effects like Hextech Ray when moving to open battlefields, since they don't gain their combat-triggered benefits.