1634, the Galileo affair
One of the problems with people from the future coming back is that people tend to imitate what other people are doing if it seems to work. You can't know everything, so if lots of people are going to a restaurant, then you will probably figure it's a good restaurant. This means that when the future types show up, that you will probably decide to imitate them.
When your enemies keep defecting to your side, it makes things kind of dull in terms of combat. If you are looking for military porn, this is not your kind of book. No modern weapons against essentially unarmed opponents, just people trying to figure our what to do when the world turns upside down.