Let's say this:
Germany has the MP18 and Mg08/15 in quantities in 1916. Maybe something like this by 1917: http://www.thefirearmblog.com/blog/2...karabiner-m35/
France has the Chauchat and the M1909 in 1915.
Britain the Lewis gun in 1915.
The Russians the Fedorov Automat in 1916.
The Austro-Hungarians their 'light pattern' Schwarzlose in quantities in 1915.
How does this affect tactics and the course of the war?
My view is that an earlier recognition by the high commands of each major power that automatics are critical to a future war causes each to support various schemes prewar, so as a result automatics make an earlier appearance in the war due to earlier investments in research and production.
As it was most had the research in place prewar historically, but they weren't supported by the cost conscious general staffs. Assuming here that they were, small unit tactics start to devolve to the squads earlier, as firepower increases. Light machine guns form the base of fire for squads, while those nations that have submachine guns (machine pistols) , use them for trench raids or for trench-breaking storm teams.
The attacker can better support offensives, but the defender now has much more mobile firepower and can more quickly transition to strongpoint defenses instead of contiguous trenches than historically.
I imagine a situation like in 1917 when the Germans first started using the MG08/15 during the Nivelle offensive; the Germans have a bunch more automatic weapons in 1916, so the Somme is bloodier and German defenses are stouter and counterattacks are better supported, meaning they suffer fewer losses during the battle. Western Allied losses were, especially in the early days, caused by machine guns. The French and British had their light machine guns in service by this point historically, so its not a likely to make as much of a difference to their attacks in this scenario. Verdun remains unchanged, as it was more an artillery battle than about automatics.
The French and British likely start developing their own submachine guns once the Germans start using their. In trench situations the MP18 is a huge advantage. As US soldiers noticed in Korea, where there was trench warfare, having a machine pistol with a high rate of fire was a massive advantage, which is why they picked up the PPSH from the ChiComs whenever possible. The MP18 fired at about 500 rpm from a drum mag and was pretty reliable; ergo in raids and patrols the Germans would be able to lay down heavy fire quickly, unlike light machine guns, which generally need to be fired prone to be accurate. This gives the Sturm Abteilungen from 1916 on (first used at Verdun) a major advantage in close combat scenarios. Especially if the Germans develop an automatic carbine like the M35.
The US would probably use their BAR and Thompson guns from the beginning if the Germans already have their versions of each.
So overall 1916 isn't quite as bloody for the Germans and 1916 and 1917 are likely bloodier for the British and French. I'm not sure if the course of the war would actually change as a result, but modern tactics likely appear even sooner, as company level maneuver becomes too costly sooner. That means the tactics that made the Somme so bloody are phased out quicker (company-level charges are instead based on squad-level fire and maneuver, with companies gaining more firepower to lay down a heavier base of fire around which everything can maneuver).
It means major changes postwar for sure.
The assault rifle would be likely pre-WW2 in many countries, which would make squad-level combat more interesting from the beginning.