Chlorine was developed into the deadlier, invisible gas phosgene by the chemist, Fritz Haber. Then in July 1917 Haber made mustard gas which looked like sherry and smelled of onions. It reacted with body fluids so could easily burn out a man’s lungs or cause foam to form on it. If it reached a sweaty patch of the body, the skin would begin to burn and blister (Fig.9). Mustard gas could be used even when the wind was blowing the wrong way since it could be loaded into a shell and fired into the areas behind the enemy’s trenches. Gas from the detonated shell was then blown back towards the artillery and quickly sank into the enemy trench. The normal British gas mask provided no protection against mustard gas, and for a time British soldiers were defenceless (Fig.10).

Although effective for a time, gas did not bring a breakthrough for either side. Gas masks kept pace with advances in gases, so that only 90,000 deaths (out of about 10 million) were caused by gas. Germany was unable to use gas much as the prevailing wind came from the wrong direction for them.

For years before 1914, farmers had been using tractors with tracks for work in muddy fields. Tracks spread the weight more evenly, stopping tractors from becoming bogged down. When a suggestion came to the Ministry of War that these tractors could be reinforced with steel plates and armed with machine guns the idea was dismissed. One Nottingham plumber who had the same idea had "the man’s mad" written all over his plans. Most British generals were cavalry men. They wanted the war to be won by their people – the cavalry. A good example of this were Kitchener’s words following the trial of the tank prototype:

Whilst the army did not seem interested, strangely the navy was. Winston Churchill, the First Lord of the Admiralty could see their value, and channelled £75,000 in developing them. To explain to his critics why the navy was developing them, he called them "Land ships." Finally, the army agreed to lend their support as long as tanks could travel at 4 mph., bridge a 2.5m wide trench and climb a 45 degree slope. At the trial of the prototype, HMS Centipede, the far-sighted Lloyd George was confident :

At the beginning, tanks only came in two varieties - the male tank , which was armed with a small artillery piece on each side of the hull (Fig.11), and the female tank, which was protected with many hidden machine guns. In later years, these two designs were combined in the same vehicle.

Tanks were first used was at the Battle of the Somme in 1916. Unfortunately, British commanders had not thought about how best to use tanks, and they were wasted. There were also too few of them in use at this stage. Once they got going they easily crossed the barbed wire and trenches, but a breakthrough was not achieved. Tank’s engines were still primitive and many broke down, ran out of petrol or were otherwise immobilised. Once this had occurred, the German artillery delivered the coup de grace.

The following year, the generals decided to try using tanks again at Cambrai. By now, the army had begun to realise the basics of tank strategy. They knew the element of surprise was essential. Three hundred and seventy-eight were secretly moved up the line where they massed, as the planes of the Royal Flying Corp flew overhead, drowning out the tank’s engine noise. The location had also been selected well. Cambrai was part of the Hindenburg Line, a very strongly defended line of trenches designed to take advantage of all natural features. The Germans regarded an attack highly unlikely. There had been little artillery fire previously, so the tanks were not hindered by shell holes and mud. The tanks rolled forward and captured four miles. However, once again, a lack of men to defend the captured trenches lost the advantage. All available tanks were used in the initial attack, so that there were no reserves when tanks broke down or were destroyed. Instead, the cavalry were ordered to charge, and were massacred by machine gun fire. The Germans had by now reorganised themselves and brought up reserves so that normal trench warfare returned.

Although to most, tanks seemed a failure, the British and French armies kept faith. This faith unusually paid off. Tanks played a crucial role in the pushing back of the Germans towards the end of the war. By now generals were really learning how to use tanks effectively. At last they realised that tanks should not advance further until the following infantry had caught up. By late 1918 engines had become more reliable, and tank design more successful (Fig.12), resulting in startling gains.