How could politicians still influence people today, with so much access to information, so people still go to war on their command?

Having information around doesn't guarantee that you are automatically informed. I'm going to use the expression "god helps those who help themselves" here.

Same way they influence people for anything else...

Access to misinformation.

The weapons of war are so lethal now. Chemicals. Larger bombs than what ended WWII.

I would hate to see what the next world war would look like.