Can Shen be played as a reaction to pump attack when moving a unit to attack a contested battlefield (a battlefield the player does not control)?

Ruling: Shen can be played as a reaction to anything that allows reactions, but can only be placed in valid locations: your base or a battlefield you control. You cannot play Shen to a battlefield you are attacking because you do not control it.

Sequence:

• Shen's reaction timing allows him to be played anytime reactions are allowed
• However, placement is restricted to locations you control (base or controlled battlefields)
• When attacking a battlefield, you do not control it, so Shen cannot be played there

Nuances:

• Shen can be played as a reaction on defense at a battlefield you control
• Shen can be played as a reaction to other effects (like Ride the Wind) to a battlefield you control
• If a card effect explicitly allows playing units to other locations (example given: Purple MF allowing open battlefields), those become valid locations for Shen as well