Izlyr warned that the hypothetical murder of the Third Doctor on Peladon would amount to a declaration of war, which would lead to Peladon's destruction at the hands of the Galactic Federation. (TV: The Curse of Peladon)
Brother Lassar of the Krillitanes asked the Tenth Doctor if he would declare war on them. (TV: School Reunion)
During the Battle of Canary Wharf, Cyber-Leader One warned Dalek Sec of the Cult of Skaro that the Daleks had declared war against the Cybermen when Dalek Thay exterminated two Cyber-Units. Sec retorted that they were not waging war, but pest control. (TV: Doomsday)
The Shadow Architect, having determined that the stolen planets were taken with hostile intent, had the Shadow Proclamation declare war "right across the universe". The Doctor, however, refused to lead them into battle as he headed to the Medusa Cascade without them. (TV: The Stolen Earth)
Brock of the Shoal of the Winter Harmony took the Twelfth Doctor's opposition to them as a declaration of war. The Doctor maintained that he was "drawing a line", going on to suggest that they "step back from it with awesome speed. (TV: The Return of Doctor Mysterio)