That's actually wrong. Robertson took over photographing the Crimean War for the British government from Roger Fenton. -Owen
First war photographer
Carol Popp de Szathmàri preceded Fenton by 1 year. In 1854 he took some of the the first war pics. And not "artistic", staged war pictures, but real ones. Right now, the order is:
- An American anonymous photographer, which took some daguerrotypes during the war between USA and Mexico, in 1847 (2 can be seen here).
- Carol Popp de Szathmàri (Hungarian-Romanian), which took photos of various officers in 1853 and of war scenes near Oltenița and Silistra in 1854, during the Crimean War (From his 200 pictures album only 9 survive today)  
- Ernest Edouard de Caranza (French) which took photos of the French army near Varna in 1854.
- Roger Fenton, 1855
- James Robertson 1855