Background
Before the first world war broke out, Germany had been building a massive and very expensive fleet. This created tensions with Britain before the war even started, as they saw it as a threat to the British navy, which was the largest and most advanced fleet in the world at the time. Out of fear that the fleet would be decimated by the British, Germany kept it in port at the beginning of the war.
In 1916 however, they started to lure stray British ships away from the main fleet and slowly chip away at the navy in an attempt to reduce the hold the Allies had over German shipping, which was already having a massive effect on the civilian population. Food prices in 1916 doubled in comparison to pre-war prices, and a year later 1000 calories was the average caloric intake per day for a German.
The Battle of Jutland
On May 28th 1916, the British deciphered a German radio message using German naval codes recovered by Russia in 1914, stating that pretty much the entire German fleet was sitting in the North Sea, so the British Navy was sent out to intercept them. As they approached on May 31st, there was some confusion over the message being correct, which resulted in both fleets idling just over the horizon from each other, with neither side knowing the enemy fleet was there.
By chance, a civilian trawler passed through both of the fleets, prompting both sides to send a scout to investigate, which lead to both the British and German navies discovering that the entire enemy fleet was almost within range to engage. The Battle of Jutland is basically the only major naval battle of the modern era. The Russo–Japanese war 10 years earlier had smaller battles with similar ships, and WW2 would also have a lot of similar small engagements throughout the war (which would also feature some of the very same ships that were present on the British side at the Battle of Jutland), but nothing would ever approach this scale again. After the battle, the German fleet commander would describe one moment of the battle by saying:
"It was now obvious that we were confronted by a large portion of the English fleet. The entire arc stretching from north to east was a sea of fire. The flash from the muzzles of the guns was seen distinctly through the mist and smoke on the horizon, although the ships themselves were not distinguishable."
In total, 250 ships were at the Battle of Jutland. The entire battle can be recreated through the accuracy of the logs that were kept by each ship, and there are some photos that still exist.
The What-If?
If that civilian trawler did not sail between both fleets, there is a chance there would have been no battle. The battle itself did not produce a huge amount of casualties, and both Britain and Germany would claim victory afterwards. It did have one huge affect that could have changed the course of the war though, and maybe even have a knock on effect for other events that would change the course of history.
After the battle, Germany realised that they could not compete with the British at sea. This prompted them to restart their campaign of sinking merchant ships with submarines on an increased scale, and would eventually lead to the US joining the war. Furthermore, if the US did not join the war, they may not have gotten involved in a number of other conflicts throughout the past 100 years. The US policy of isolation was shattered by Germany sinking ships with American citizens onboard, and it has not been regained since.
Another note to add is that the imminent US involvement in the war prompted Germany to begin the Kaiserschlacht, the last German offensive of the war, which resulted in 1.5 million casualties.
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