Vast we once were

X never marked a particular spot for us

Merciless we claimed much!

It is true that the Republic of Rome was vast under the general rule of Julius Caesar ("Veni, vidi, vici").  But, perhaps the clue refers to Imperium Romanum (the Roman Empire) with its much more vast conquest and control of foreign lands by Julius' nephew, Augustus Caesar. 

(The leading letters of each line are all Roman Numerals , though the "M" is anachronisitic as a Roman Numeral for either the historical period of Julius or Augustus.)