Fullmetal Alchemist Watch Order: Brotherhood or 2003?
Two complete adaptations of one manga: Brotherhood is the author's story, 2003 is a sanctioned alternate timeline with its own ending. Which first, and what it costs.
Fullmetal Alchemist has two complete adaptations of the same manga, made six years apart, and which to watch first is the rare watch-order question with real stakes. The short answer: Brotherhood is the manga's story; the 2003 series is a deliberate alternate version. Watch Brotherhood for the story everyone means by Fullmetal Alchemist, and watch 2003 afterwards if the world holds you, because nothing about it is a rough draft.
Why two versions exist at all
When the first anime premiered in October 2003, roughly five volumes of a manga that would run to twenty-seven existed. The studio was adapting a story whose ending its author had not written, and rather than pad or pause, they built their own back half: with the author's blessing, the 2003 series follows the manga until the death of Maes Hughes and then walks off the map entirely, inventing its own villains, its own cosmology for the homunculi, and its own ending, completed by the 2005 film Conqueror of Shamballa.
Brotherhood arrived in 2009, once the manga was nearly done, and adapted the real thing across 64 episodes.
What that means in practice
The two shows share their first act and almost nothing after it. In 2003, each homunculus is the residue of a failed human transmutation, an alchemist's grief given a body; in Brotherhood they are manufactured by a single villain with a plan. The tones diverge the same way: 2003 is a tragedy about consequences that stays close to the brothers' guilt, Brotherhood is a war story that widens to a continent. These are different arguments built on the same opening, which is why the usual dismissal of one or the other misses the point.
The one real cost of Brotherhood-first
Because Brotherhood assumed its audience had seen 2003, it moves through the shared early material at a sprint; the Tucker episode and the Hughes arc land harder in the older show, which had time to sit in them. That is a genuine loss and the standard advice accepts it, because the alternative, watching 51 episodes and a film to reach an ending the manga's author never wrote, is a stranger first exposure to the story.
The order, stated plainly
Brotherhood, all 64 episodes. Then, if you want the other timeline: Fullmetal Alchemist 2003, all 51, followed by Conqueror of Shamballa, which is that continuity's actual ending. The Sacred Star of Milos is a standalone side film in Brotherhood's world and can slot anywhere after its midpoint or nowhere. Nothing requires both series. The manga's readers got one story; 2003's audience got a different one; you are the rare viewer who gets to choose the order of universes, and either one alone is complete.