Allied engineers worked hard to build twenty pontoon bridges over the River Mehaigne so that the army could cross with its equipment. As soon as French scouts became aware of the operations, they sent urgent dispatches to Marshal Villeroi. He responded by marching a substantial part of his army — 40,000 soldiers, in all — to a position between Merdorp and Namur. The first part of Marlborough's plan had worked perfectly. He had read the French commander's mind like a book. Instead of being distracted by what he assumed was a deliberate feint in the north, Villeroi hastened to repel an apparent attack in the south. He had swallowed the bait dangled so temptingly before him.

Marlborough acted promptly. Gathering his army of British, German and Danish troops, he hurried them north through the night in the direction of Elixhem. The advance was led by General Ingoldsby and Count Noyelles with 38 squadrons and 20 battalions supported by 600 pioneers. The cavalry carried large trusses of hay to serve as makeshift fascines when they met ditches or rivulets. Wider streams compelled them to make diversions.

Many complained about the rigours of a forced march through heavy mist and persistent drizzle but Captain Daniel Rawson was not one of them. Riding as part of Marlborough's staff, he was aware of the genius behind his commander's strategy. A double bluff had been used. Because there had been a feint near Leau in the north, Villeroi had been tricked into believing that the real danger lay in the south and the whole Dutch corps — with the exception of Marshal Overkirk — had also been misled. When they crossed the Mehaigne, they thought that they truly were the main strike force against the French. In fact, they merely acted as a decoy and would soon receive orders to withdraw.

By dawn on 18 July, Marlborough had reached his destination, a section of the Lines where the topography greatly favoured the French and where it had been reinforced with a series of fortifications.