In the Galaxy RangersFan FicChrysalis, the League fleet is in a hopeless battle in the Queen's asteroid field, having been forced into a trap. However, they some critical backup courtesy of the otherwise-xenophobic Traash (which even Her Majesty won't engage because they're just that nasty if crossed) and the Circle of Thought (who were isolationists, content to let the galaxy tear itself apart, so long as they were left alone...until Niko showed them that the Queen was an exile from the Circle, and thus a mistake they made).
The Mega Crossover fic Odyssey, by Drunkengrognard, has the likes of Chrono Harlaowan and Jean-Luc Picard, amongst others, preparing their ships for a major battle against a group of Voidspawn (essentially giant killer space squids) and planning to close the portal that they are coming from. Due to circumstances connected to the portal opening, their current location has basically turned into a multiversal thoroughfare; the suggestion is made that help could be requested throughout said multiverse in order to ward off the Voidspawn long enough for the portal to be shut. Rogue Squadron and the fleet they were a part of are amongst the first to answer the call, and it just gets bigger from there.
During the Final Battle of the MLP AU fic Rainbooms and Royalty, the Diamond Dogs, sea ponies, and gargoyles that the Mane Six had befriended on their way through the Everfree Forest all show up to help them fight Nightmare Moon's Shadowbolt army.
In book 2 of A Growing Affection, when Naruto is captured by the Akatsuki Tsunade immediately contacts the other four Kages to request aid. The Kazekage and Mizukage send troops to assist, while the Tsuchikage and Raikage only provide information. And the Raikage's assistance is especially grudging.
A flashback to the Final Battle of the Age of Myths (G1) shows that both sides did this — Hydia called in her entire family of witches and warlocks, while the Paradise Estates ponies called on every ally they had.
Probably the biggest example comes in City by the Bay, when Xander has been infected by a demonic virus and summons mental versions/soul fragments of everyone he's been friends/allies with over the course of the series thus far, and manages to drag along everyone they're closely tied to as well, to help him fight off the demons trying to posses him. Made all the more amusing by the fact that there had just been a backwards time-jump that removed the memories of most of those he called of ever having met him in the first place, leading to some interesting WTF moments.
In Waking Nightmares'Pony vs Machine arc, Ponyville calls for help from Chrysalis, the Wonderbolts, Mare-Do-Well, and virtually all of Ponyvile (whom are civilains) to defend Ponyville from Gray Mann's robot army, which is at least a thousand strong.
In The Measure Of A Titan the Big Bad goes to great lengths to avert this by arranging things so that every potential ally the Teen Titans could call is tied up elsewhere. The Justice League is off-planet, the other Titans are dealing with other villains, and even Tameran has been attacked by the Gordians.