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Why did the French build forts in the Ohio River valley?

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Painting of Edward Braddock and British troops in uniform advancing through dense forest, facing Native American and French fighters during the French and Indian War near Fort Duquesne. — French and Indian War British commander Edward Braddock and his troops preparing to march on the French-held Fort Duquesne (now Pittsburgh, Pennsylvania) during the French and Indian War.

The French built forts in the Ohio River valley primarily to consolidate their strategic position and assert control over the region against British expansion.1

Following a policy shift initiated by the governor of New France, the Marquis de La Galissonière, the French sought to secure the valley to protect their fur trade and confine British colonists to the east of the Allegheny Mountains. By establishing a line of fortified posts, the French aimed to enforce their territorial claims—which were based on the explorations of René-Robert Cavelier, sieur de la Salle—and effectively drive out British traders who were viewed as trespassers on French lands. This military buildup was a direct response to the growing presence of settlers and traders from Virginia and Pennsylvania, ultimately serving as a catalyst for the French and Indian War.2314