I am quoting from The Memoirs of Sherlock Holmes, The Final Problem by Arthur Conan Doyle:
"Because you will find me a dangerous companion now. This man's occupation is gone. He is lost if he returns to London. If I read his character right he will devote his whole energies to revenging himself upon me. He said as much in our short interview, and I fancy that he meant it. I should certainly recommend you to return to your practice."
It was hardly an appeal to be successful with one who was an old campaigner as well as an old friend. We sat in the Strasburg salle-à-manger arguing the question for half an hour, but the same night we had resumed our journey and were well on our way to Geneva.
I found the word in every online dictionary. For example, Collins defines as:
A campaigner is a person who campaigns for social or political change.
which is basically "militant" but it feels like the author meant that he was an old soldier but I couldn't find this meaning anywhere.