when wasteful wars shall statues overturn, …

Giving you the complete explanation of this stanza with meanings.

When wasteful war shall statues overturn,

And broils root out the work of masonry,

Nor Mars his sword nor war's quick fire shall burn

The living record of your memory

broils: tumult, fighting, disturbances, esp. in war

work of masonry: refers to monuments built by masons

Mars: the god of war

Sword: a weapon with a long metal blade

quick: lively, fast moving, searching out living record: this written memory of your life which continues after you are dead

The poet says that wars are wasteful as they destroy the statues built by the royal men. Such fights are destructive as they destroy the great monuments and turn them into ruins. This poetry that has been written in the memory of the brave man cannot be destroyed even by the greatest warrior Mars. It will survive through all the wars.