What’s the best way to connect or merge two separate Active Directory domains?

This is very doable, but it’s worth bringing in an experienced Microsoft partner or migration consultant. They can compare a forest or domain trust, a full domain migration, and a hybrid Entra ID design, then build a staged plan. The details around DNS, duplicate usernames or UPNs, SID history, workstation rejoining, application dependencies, and permissions are where migrations commonly become painful.

If you’re already considering a major migration, evaluate moving the subsidiary into the parent organization’s Microsoft 365 and Entra ID tenant. You could start with a cloud or hybrid identity setup and potentially move more services away from on-premises AD over time. This can simplify identity management, but it still requires planning around synchronization, devices, applications, licensing, and user transitions.

An Active Directory trust is probably the least disruptive starting point. You’ll need reliable connectivity between the sites, usually through a site-to-site VPN or equivalent, along with careful DNS, routing, authentication, and security-group planning. A trust lets the parent domain grant access to users from the subsidiary without immediately migrating every account and workstation. Plan the design and permissions carefully, since cross-domain groups and resource access can become complicated.

Before deciding between a trust and a migration, inventory the users, workstations, applications, file shares, authentication requirements, and compliance constraints. A trust may be the best short-term solution, while a properly planned migration could simplify administration in the long term. The right choice depends heavily on the existing environments and how much independence the subsidiary needs.