Many books have been written on this first major industrialized world war in human history, known in history as the Great War, the war to end all wars and in many ways the issues and causes that provoked the conflict in August 1914 are mirrored today in the Ukraine.
The first cause that serves as a major catalyst for comparison is treaties. In 1914 as today, there were distinct groups of countries banded together for common self defense reasons. The trigger point for WW1 to escalate was the Germans violating the neutrality of Belgium. Because Britain had guaranteed this, it became reason Britain joined France against Germany.
One could effectively argue Russia violated Ukrainian neutrality but key difference in our view was methods. Germany rolled right into Belgium under a battle plan created in 1905 known as the Schieffen Plan and the army was mobilized on a new railroad system and struck quickly and by surprise whereas Putin announced his intentions with congregated mechanized formations grouped on border so no surprise there as the units stood by for weeks until they attacked.
Seeing as the fake news being disseminated from both sides really clouds what is really happening on the battlefield, suffice to say the second common denominator is nationalism. Both sides are framing their narratives as survival of their countries , peoples and way of life.
In WW1, Russia came to the defense of Serbia, who was Slavic in origin after one of their own whacked the Duke Ferdinand from Austria Hungary and which sparked the whole thing to start with. Seeing as The Czar was with the Allies even though he was first cousin by blood to the pompous Kaiser Wilhelm of Germany of the Central Powers (as he was also with King Geoge the 5th of England) thru the late Queen Victoria of England, this set the table for the war on the Eastern Front.
It is important to note WW1 saw the decimation of 4 empires by it’s conclusion, The Ottomans in Turkey, Czar in Russia, Hapsburgs in Austria Hungary and the Kaiser in Germany. Excepting The Kaiser whose reign began in mid 1870s with his father and he took over in 1880s upon his assassination, the other 3 empires had been in existence for HUNDREDS of YEARS so their demise signaled a sea change in the rule of royalty in Europe and was the birth of communism, socialism and facism which sprouted among the masses in the post war period.
If one is to truly explore the history of the causes of WW1 that dwarf this post and how they dovetail with Ukraine today, the outstanding book “ARMS OF KRUPP” by William Manchester is a great resource. I also recommend watching the movie “GUNS OF AUGUST” posted here