| Conflict: | Fifth Battle of Ypres |
| Partof: | the Western Front of the First World War |
| Date: | 28 September – 2 October 1918 |
| Place: | Ypres, Belgium to Ghent, Belgium |
| Coordinates: | 50.9003°N 3.0211°W |
| Result: | Allied victory |
| Combatant1: |  Belgium |
| Commander1: | King Albert I Cyriaque Gillain Herbert Plumer Jean Degoutte |
| Commander2: | Erich Ludendorff Crown Prince Rupprecht Sixt von Armin |
| Strength1: | 28 divisions |
| Strength2: | 16 divisions |
| Casualties1: | British: 4,685 Belgian: 4,500 Newfoundland 15 |
| Casualties2: | 10,000 POW 300 guns 600 machine guns |
The Fifth Battle of Ypres, also called the Advance in Flanders and the Battle of the Peaks of Flanders (French: Bataille des Crêtes de Flandres) is an informal name used to identify a series of World War I battles in northern France and southern Belgium (Flanders) from late September to October 1918.
After the German spring offensive of 1918 failed to achieve a decisive victory, German morale waned and the increasing numbers of American soldiers arriving on the Western Front gave the Allies a growing numerical advantage over the western armies of the German Empire. To take advantage of this, Marshal of France Ferdinand Foch developed a strategy which became known as the Grand Offensive, in which attacks were made on the German lines over as wide a front as possible. Belgian, British and French forces around the Ypres Salient were to form the northern pincer of an offensive towards the Belgian city of Liège. The British Second Army had followed up some minor withdrawals and had fought the action at Outtersteene Ridge on 18 August, after which there was a lull. Allied troops in the area were well rested by late September.
The French: Groupe d'Armées des Flandres (GAF, Flanders Army Group, King of the Belgians) attacked at on 28 September, after a three hour artillery preparation. The GAF attacked with divisions, divisions of the Second Army and divisions of the Sixth Army. The British attacked on a 4.5miles front up to the Ypres–Zonnebeke road, from where the Belgian army attacked on a line north to Dixmude. The Allied attacks quickly penetrated the German defences and advanced up to 6miles. Much of the ground west of Passchendaele, abandoned during the withdrawal of early 1918, was recaptured. Rain began to fall but by the evening the British had taken Kortewilde, Zandvoorde, Kruiseecke and Becelaere; Belgian troops had captured Zonnebeke, Poelcappelle, Schaap Baillie and Houthulst Forest. On the southern flank, minor operations by three British divisions advanced to St. Yves, Messines and the ridge from Wytschaete to Hollebeke. The German front line ran from Diksmuide to Houthult, Becelare, Zandvoorde and Hollebeke.
Messines, Terhand and Dadizeele fell on 29 September and by the next day, despite the captured ground becoming another slough of mud, all of the high ground around Ypres was occupied by the Allies. By 1 October, the left bank of the leie (Lys) had been captured up to Comines and the Belgians were east of a line from Moorslede to Staden and Diksmuide. The advance continued until 2 October when German reinforcements arrived and the offensive outran its supplies. Due to the state of the ground, were delivered by parachute from and British aircraft.
The British suffered the Belgians "nett" casualties from among and ill or wounded. The Allies advanced up to 18miles, with an average advance of 6miles and captured prisoners, and
The offensive was continued with the Battle of Courtrai (14–19 October).
The Allied units of Army Group Flanders (King Albert I of Belgium), had the French General Jean Degoutte as Chief of Staff.
Army Group Rupprecht of Bavaria (Crown Prince Rupprecht of Bavaria), commanding the northern German army group, held Flanders with the 4th Army, which had less than five divisions in the area.