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)
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)