On February 9th he had reached Phillipolis, well ahead of the British pursuit, and spent a day or two in making his final arrangements before carrying the war over the border. His force consisted at this time of nearly 8000 men, with two 15-pounders, one pom-pom, and one maxim. The garrisons of all the towns in the south-west of the Orange River Colony had been removed in accordance with the policy of concentration, so De Wet found himself for the moment in a friendly country.
The British, realising how serious a situation might arise should De Wet succeed in penetrating the Colony and in joining Hertzog and Kritzinger, made every effort both to head him off and to bar his return. General Lyttelton at Naauwpoort directed the operations, and the possession of the railway line enabled him to concentrate his columns rapidly at the point of danger. On February 11th De Wet forded the Orange River at Zand Drift, and found himself once more upon British territory. Lyttelton's plan of campaign appears to have been to allow De Wet to come some distance south, and then to hold him in front by De Lisle's force, while a number of small mobile columns under Plumer, Crabbe, Henniker, Bethune, Haig, and Thorneycroft should shepherd him behind. On crossing, De Wet at once moved westwards, where, upon February 12th, Plumer's column, consisting of the Queensland Mounted Infantry, the Imperial Bushmen, and part of the King's Dragoon Guards, came into touch with his rearguard. All day upon the 13th and 14th, amid terrific rain, Plumer's hardy troopers followed close upon the enemy, gleaning a few ammunition wagons, a maxim, and some prisoners. The invaders crossed the railway line near Houtnek, to the north of De Aar, in the early hours of the 15th, moving upon a front of six or eight miles. Two armoured trains from the north and the south closed in upon him as he passed, Plumer still thundered in his rear, and a small column under Crabbe came pressing from the south. This sturdy Colonel of Grenadiers had already been wounded four times in the war, so that he might be excused if he felt some personal as well as patriotic reasons for pushing a relentless pursuit. On crossing the railroad De Wet turned furiously upon his pursuers, and, taking an excellent position upon a line of kopjes rising out of the huge expanse of the Karoo, he fought a stubborn rearguard action in order to give time for his convoy to get ahead. He was hustled off the hills, however, the Australian Bushmen with great dash carrying the central kopje, and the guns driving the invaders to the westward. Leaving all his wagons and his reserve ammunition behind him, the guerilla chief struck north-west, moving with great swiftness, but never succeeding in shaking off Plumer's pursuit. The weather continued, however, to be atrocious, rain and hail falling with such violence that the horses could hardly be induced to face it. For a week the two sodden, sleepless, mud-splashed little armies swept onwards over the Karoo. De Wet passed northwards through Strydenburg, past Hopetown, and so to the Orange River, which was found to be too swollen with the rains to permit of his crossing. Here upon the 23rd, after a march of forty-five miles on end, Plumer ran into him once more, and captured with very little fighting a fifteen-pounder, a pom-pom, and close on to a hundred prisoners. Slipping away to the east, De Wet upon February 24th crossed the railroad again between Krankuil and Orange River Station, with Thorneycroft's column hard upon his heels. The Boer leader was now more anxious to escape from the Colony than ever he had been to enter it, and he rushed distractedly from point to point, endeavouring to find a ford over the great turbid river which cut him off from his own country. Here he was joined by Hertzog's commando with a number of invaluable spare horses. It is said also that he had been able to get remounts in the Hopetown district, which had not been cleared--an omission for which, it is to be hoped, someone has been held responsible. The Boer ponies, used to the succulent grasses of the veld, could make nothing of the rank Karoo, and had so fallen away that an enormous advantage should have rested with the pursuers had ill luck and bad management not combined to enable the invaders to renew their mobility at the very moment when Plumer's horses were dropping dead under their riders.