return home at once or resign his appointment, Simmons, who had outstayed his leave, sent in his resignation, which was accepted on 30 June 1854. When at the end of March the Western powers allied themselves with Turkey against Russia, Simmons was formally attached to Omar Pasha's army on the Danube as British commissioner. He gave advice and help in the defence of Silistria, which he left during the siege on 18 June to join Omar Pasha and the allied generals at Varna. Five days later the siege of Silistria was raised, and the generals at Varna decided that Omar Pasha should take advantage of this success to cross the river and attack the Russian army at Giurgevo.
On 7 July Simmons was in command of 20,000 men of all arms at the passage of the Danube and the battle of Giurgevo. He threw up the lines of Slobodzie and Giurgevo in presence of the enemy, who tried to prevent him, while a Russian army of 70,000 men lay within seven miles. For his services with the Turkish army and his share in the defence of Silistria and the battle of Giurgevo, when the Russians were routed, Simmons was promoted brevet major on 14 July 1854, and given the local rank of lieutenant-colonel (a brevet lieutenant-colonelcy following, 12 Dec). During the retreat of the Russians and the occupation of Wallachia by the Turks, Simmons was frequently in charge of re-connaissances upon the enemy's rear until they had evacuated the principality.
In the meantime the allies had invaded the Crimea, the battles of the Alma, Balaclava, and Inkerman had been fought, and the siege of Sevastopol was in progress. Simmons opposed Napoleon III's proposal that the Turks should advance on the Pruth so as to act on the Russian line of communications with the Crimea. Realising the weakened condition of the allies after Inkerman and that there were no reserves nearer than England and France, he urged that the Turkish army should reinforce the allies in the Crimea. After much discussion the advanced guard of the Turkish army in Jan. 1855 occupied Eupatoria, which Simmons at once placed in a state of defence, in time to repulse a determined attack by the Russians on 17 Feb. The Russians were 40,000 strong, while the Turkish garrison was small. After this action the remainder of the Turkish army arrived from Varna, and Simmons laid out and constructed an entrenched camp. From April to September 1855 he was with Omar Pasha's army before Sevastopol, taking part in the siege until the place fell. He was created C.B. on 13 Oct.
When after the fall of Sevastopol Omar Pasha took his army to Armorica to operate against the Russians south of the Caucasus, and thus relieve the pressure on the fortress of Kars invested by the Russians, Simmons continued with him as the British commissioner. Omar, advancing into Mingrelia with 10,000 men, encountered 12,000 Russians on the river Ingur on 6 Nov. 1855. Simmons commanded a division which, crossing the river by the ford of Ruki and turning the Russian position, captured his works and guns and compelled the enemy to retreat. The casualties were small, so sudden and unexpected was their turning movement, the Russians losing 400 and the Turks 300 in killed and wounded. Omar Pasha in his despatch attributed the success mainly to Simmons. Unfortunately the campaign began too late to enable the relief of Kars to be effected. It capitulated on 26 Nov.
Early in 1856 Omar Pasha sent Simmons to London to explain his views for the next campaign in Asia Minor, against Russia, but, by the time he arrived in England, peace negotiations were in progress, and the treaty of Paris was signed on 30 March. For his services Simmons received the British war medal with clasp for Sevastopol ; the Turkish gold medal for Danubian campaign, and the Turkish medal for Silistria ; the third class of the order of the Mejidie (the second class was sent by the Sultan, but the British government refused permission for him to accept it on account of his rank) ; the Turkish Crimean medal ; the French legion of honour, fourth class ; and the Sultan of Turkey presented him with a sword of honour and made him a major-general in the Turkish army. In his service with the Turkish army Simmons had shown a knowledge of strategy and a power of command which should have led to further command in the field, but did not.
In March 1857 he was nominated British commissioner for the delimitation of the new boundary under the treaty of Paris between Turkey and Russia in Asia. Major-general Charles George Gordon [q. v.] was one of three engineer officers who accompanied him as assistant commissioners. The whole frontier from Ararat to the Black Sea was traversed and questions of principle were settled by the commission ; the actual marking of the boundary line was carried out by their expert assistants in the following year