In Wonder Woman (2017), Steve Trever brought Diana to main battlefield, but stopped her from going to No Man's Land. She didn't obey his words and move forward to No Man's Land. What was Steve Trevor's plan, if not taking her to No Man's Land?
We get a short insight into this in the film's Official Novelisation. He wanted to stick to their original plan, the reason they came. Diana on the other hand wanted to cross No Man's Land to save a French town which had been enslaved.
“The woman pointed beyond the trench, into the indeterminate distance. Diana turned to Steve. “We need to help these people.” Steve looked across no man’s land and shook his head. “We need to stay on mission.” The Chief agreed with Steve. “And there’s no safe crossing for a least a day away.”
Wonder Woman: The Official Movie Novelization
As for why they were there. Steve had not intended to go to the trenches. He planned on traversing the land through the backroads, however as Diana insisted, he took her to the Front Lines
“You wanted me to take you to the war. This is it. Front lines,” he said grimly.
The impression I got from watching that part of the movie was that he (along with the rest of the party, not counting Diana) had intended a covert infiltration past the German lines during a lull in the fighting (or an assault at a nearby section of the trenches that would draw fire there). In fact, however, I don't recall thinking Steve had an actual plan, so much as a half-baked idea.