One impact of the printing press was a general increase in literacy and education. This was because books suddenly became cheaper - until then they had to be hand-written, and so the monopoly on knowledge was generally in the hands of the church, who was responsible for re-writing books.
One indirect impact was the decrease of the power of the church, who no longer had the monopoly on knowledge.
This is most likely specifically asking for World War I.
The first thing is rather not a gain, but a trust. Most of Europe, Asia, as well as the Middle East and Africa, were split into two camps. These camps were alliances, known as the Central Alliance (the German & Austrio-Hungary camp), and the Allies (The British & French camp).
The war was triggered by the assassination of Archduke Franz Ferdinand and his wife Sophie by a Serbian Nationalist who wanted to see a free Serbia. While Serbia was open to paying for reparation, Austrio-Hungary threw away the table in hopes of conquering and claiming Serbia for their own. The failure for the gears of war to stop before then was what caused the countries to go at war. Each country was bound by their alliance contract to help their members at need, and so it set off a chain reaction that ensued into a global war.
They had nothing to gain, but to keep their own honor, the survivability of their state, and the trust of their allies.
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It established the principle that everyone is subject to the law. Even the king. It also guaranteed the rights of individuals, and the right to a fair trial.
