It happened in my alliance today.
20k mm was sent to attack a member's 6th city from attackers 1st city.
Defending cities troops were defending.
Attack was defeated, but killed ~20k scouts.
100k mm was sent to attack the same city from the same city.
Defending city was reinforced by 100k hcav from another member's 1st city.
Attack was defeated, but not before killing all remaining scouts (~180k)
Reinforcements bounced back.
Nothing more complicated than that.