The Mustering of the Dwarves was a term used to describe the Dwarven preparation for war.
Throughout the ages of Middle-earth, the dwarves have mustered great forces for the wars they participated in such as the Nirnaeth Arnoediad and Last Alliance of Elves and Men, but the term was most famously applied to the War of the Dwarves and Orcs.
After hearing of Thrór's murder in Moria in TA 2790, Thráin, his son decided to seek revenge against his father's murders and began to prepare for a great war. He sent messengers bearing the ill tale north, east, and west to all the Dwarven Folk that would aide in the settling of scores. The Mustering of the Dwarves took three years to assemble (TA 2790 - TA 2793) and from all over Middle-earth came many dwarves, exiles from the Lonely Mountain, dwarves from the Iron Hills under and Grór and his son Náin, and quite possibly other clans of dwarves from the Orocarni (Red Mountains). When the Mustering was complete, the host set out on a great war that killed many dwarves but nearly wiped out the orcs infesting the Misty Mountains for years to come.