Picture this, a united Scandinavia (me), France, and the UK, against Germany, Russia, and some other minors, it's about 1905ish. The war began with Germany attacking me to get back the northern half of Holstein I had taken a decade or so ago when they were at war with France. My first move is to send the army I have guarding the border with Germany into Kiel, and at first Germany sends everything against France and all I'm facing is around 100,000 troops, but while I'm juggling troops along the Russian border, I notice that the battle in Kiel has jumped from 150k-100k to 150,000 vs 350,000. I pause and mobilize, sending a just-finished stack to the Russian border, the first clump of reserves arrive in my army center and I send them off, 350,000 to 550,000, another wave and we're neck and neck at around 500k each. France starts making headway while the UK does its usual thing of covering every province in Britain with troops. I then get a notice that the USA has joined the Great War... on our side! Naturally they immediately set about doing nothing, seeing as I already have naval superiority thanks to my and the UK's fleets. The war goes on for another year or so, I forget, but when peace is declared, that battle in Kiel was still raging. I won and actually got most of Germany's colonies. Less than half a year later I invent tanks, better late than never?
I'm not completely certain as to the casualties of the battle at Kiel, but I fed all but around 100,000 of my reserves into it and I was getting nervous as to whether my army would hold nearing when peace was concluded. I also noted a battle between France and Germany that put the participating German troop total at over 1,000,000, and France was roughly equal.
A million troops from both sides? When did a battle like that happen in real life?