Early-life human gut microbiome assembly can follow different trajectories, with breastfed infants often harboring Bifidobacterium-dominant communities. While the dominance of Bifidobacterium is associated with health benefits, alternative communities dominated by Bacteroides/Phocaeicola (Bacteroides group) are also common in industrialized countries, and the ecological mechanisms underlying this divergence remain unclear. Here, we combine experimental, theoretical, and in silico data analyses to demonstrate how priority effects and facilitation between Bifidobacterium species can influence microbial community composition in the breastfed infant gut. We performed culturing experiments in a medium containing human milk oligosaccharides (HMOs) to examine the competitive dynamics between Bifidobacterium breve and Bacteroides fragilis, both in the presence and absence of Bifidobacterium bifidum. We found that in the absence of B. bifidum, B. fragilis consistently outcompeted B. breve. However, when B. bifidum was present early, community outcomes were affected by priority effects, as B. bifidum improved the fitness of B. breve by providing HMO-degradants. Metabolite profiling further showed that Bifidobacterium-dominant communities produced higher levels of lactate and formate, potentially suppressing B. fragilis growth. The facilitative effect of B. bifidum is also seen in an analysis of publicly available fecal microbiome data from breastfed infants, which revealed that the presence of B. bifidum led to a significantly higher B. breve-to-Bacteroides group ratio, suggesting that B. bifidum may promote the persistence of B. breve in the infant gut. Together, these results highlight how facilitative interactions and colonization history can jointly structure the infant gut microbiome, offering insights for informing early-life microbial interventions.