1. To gain in scale, scope, or strength in opposition to someone or something. The evidence has continued to mount against the disgraced CEO ever since the first allegations of fraud surfaced over a month ago. A rebel insurgency has begun mounting against he tyrant king.

2. To instigate, prepare, or set in motion some action or movement in opposition to someone or something. We're mounting a counter-offensive against the enemy's position in Normandy. The prosecution has mounted an aggressive and airtight case against the defendant.