The first key reason why the Germans were defeated is because of the failure of the Ludendorff offensives (Spring 1918) - Walsh p.32.

The Ludendorff offensive.
Source A.
The situation in Russia makes it possible to deliver a blow on the Western Front in the New Year. Our general situation requires that we should strike at the earliest possible moment before the American can throw strong forces in.
General Ludendorff.

Allied strengths.
Source B.
The French Field Marshal Foch was made supreme commander which meant that he could control and co-ordinate all the Allied troops on the Western front. Their combined efforts brought success.
From a modern history textbook.

The condition of the German army.
Source C.
The physical exhaustion of the men was so great that they could not fire their rifles. They let themselves be wiped out almost without caring or moving. We are glad if the ration carts can get up to us at might - then the men and horses feed for the next 24 hours at one sitting. All the water we get comes from the icy shell holes. There was looting. I see men carrying hens under their arms, men wearing top hats, men carrying wine bottles, men who could hardly walk.
A German soldier recalling 1918.

Absence of new technology in the German forces.
Source D.
We had no tanks ... our attacks succeeded without, says Ludendorff. It is not precisely true that the Germans had no tanks at all. but they never employed move than 13 on a single occasion. Not only did they not produced tanks for their great offensive. Armoured cars were equally absent and to the best of my knowledge so were motorised machine guns (carried in motorcycle sidecars) which the British certainly possessed.
A German soldier recalling 1918.

The role of the US.
Source E.
In the summer of 1918 the Allies received a great boost to morale when the American troops arrived.
They passed in columns closely packed in motor lorries, bareheaded, bare chested, singing American songs at the tops of their voices, amid the cheers of onlookers. These magnificent youths from across the sea, radiating health, produced a great effect.
A French officer recalling the Western Front in 1918.

Differing views of the role of tanks.
Source F.
Of the 38 tanks that went into action on the 11th all need overhauling; the crews were completely exhausted. the pulses of the view were taken immediately they got out of their tank. The beats averaged 130 to the minute or twice as fast as they should have been. Two men temporarily lost their reason and had to be held back by force. one tank commander became delirious. In some cases where infantry were carried in the tank they fainted within three quarters of an hour from the start. It is clear that the tank was not a war-winning weapon.
From a modern history textbook.

Source G.
The enemy made use of tanks in large number. they suddenly emerged from smoke clouds. Our men were unnerved. Tanks broke through our front lines causing a panic which upset our antitank guns and our artillery speedily put an end to them, but the mischief was done and solely due to the success of the tanks we have suffered enormous losses...”
A German soldier on the role of the tank in the 1918 offensives.

The state of Germany.
Source H.
We have no meat, potatoes cannot be delivered because were are short of 4000 trucks a day. Fat is unobtainable. The shortage is so great it is a mystery to me what the people of Berlin live on. The workers say ‘better a horrible end than an endless horror.’
A member of the German government in October 1918.

Source I.
The supreme command demands an immediate despatch of a peace offer to our enemies. There no longer exists any hope of forcing peace on our enemies. The enemy can bring in new and fresh reserves/ The German army hold fast and repulses all attacks with success. But we must stop the fighting to save the German people further useless sacrifices.
A letter from Commander in Chief Hindenburg to Prince Max of Baden, the German chancellor.

The role of the war at sea.
Source J.
Britain's greatest contribution to victory over Germany in the First World War was the blockade set up by the Royal Navy. The German High Seas fleet attempted to break out of the blockade at Jutland in 1916 but was defeated in battle by the British fleet. This failure led to starvation in Germany and it was lack of food that led to rebellion in Germany in 1918.
From a book written by a British naval historian written in 1983

Source K.
On 21 November 1918, under the terms of the Armistice agreement, the most powerful units of the German High Sea Fleet surrendered to Admiral Beatty, off the Firth of Forth.
The Allied victory on land had only been possible because of Allied command of the sea. Only because of the ability to use the seas had the armies that were victorious in 1918 been supplied and maintained.
The various other land campaigns around the world also depended on the use of the sea. Sea power had maintained the Allied nations in food, fuel and raw materials. Conversely it had starved the Central Powers of all kinds of supplies, in a ruthless blockade that had contributed to their final collapse.
Dr. Eric Grove.

Germany loses her allies.
Source L..
The Germans were hit by the news of the defeats for their friends. Greece took sides against Germany and Allied armies there could now advance. By 25 September Bulgaria asked for peace.
In Palestine Turkish forces were retreating from he Arabs and the British. In Italy the last Austrian army lost to the British and the Italians.
From a modern history textbook