millitary leaders at the of ww1 estimated the war would last a few days a couple of weeks years decades

Many politically powerful military leaders expected World War I to last until Christmas of 1914. They never expected that it would last for four years.

Explanation:

From July 1914, the German and Britain soldiers have been waging war in western front and they expected by the Christmas time the war would end. Christmas cheer made the British, German and American soldiers to drop off their weapons and party with the fellow soldiers who were in their lines defending and fighting for the country.

The soldiers sang carols and exchanged gifts with their friends and foes in the battle field. The festive spirit made the soldiers to plunge into festive mood and expected that war would be over. But they never expected that it would last for four years.