The attackers scrawled the word 'cowards' and 'scum of the earth' in red paint on the memorial in the Duchess of Hamilton Park, in Motherwell.
Police are investigating the crime which is thought to have taken place between Saturday and Monday.
The memorial has the names of over 1100 people who died in North Lanarkshire in both the First and Second World Wars.
The word 'rats' was written in block capitals above a carved inscription commemorating lives lost in Palestine.
The memorial features inscriptions of the countries and regions that were the focus of both world wars, including France, Belgium, Jutland, Gallipoli, Salonica, Mesopotamia, Palestine and the Falklands.
It also features a sword within a wreath and a Motherwell crest flanked by two Scottish soldiers standing at ease.
Beside each of them is a kneeling soldier holding out a flag.
North Lanarkshire Council urged anyone with information to contact the police.
A post on its Facebook page said: "We're very sad to learn that the war memorial at Duchess of Hamilton Park in Motherwell has been vandalised."This disrespectful act can't be tolerated."
A Police Scotland spokesman added: "Between Saturday, June 8 and Monday, June 10, the war memorial and name plates were vandalised by graffiti at Duchess Park, Avon Street, Motherwell."