If in air-naval combat, the reaction asault the ground unit loose old the friendly naval units and the offensive have naval forze in the batle. The reaction ground unit loose one step and move post battle novement or the unit finish the asault move, and participates in the ground combat??
"7.45 Amphibious Assault (Offensive and Reaction)
Important: If Amphibious Assault ground unit(s) are without an accompanying friendly naval unit and opposing naval forces of any type end their movement in the battle hex as part of reaction movement, the assault is cancelled and the battle is considered lost. Each ground unit takes a one step loss and, if not eliminated, each unit then conducts post battle movement from the hex."
The reaction ground unit participates in the ground combat. Note that the rule refers to the Offensive units, not the reaction units. In essence the reaction units were actually at the location prior to the offensive representing an intelligence failure.