Is it okay to let a baby cry sometimes?

Responding to a baby's needs consistently builds secure attachment, and that is well established. It is also true that a parent putting a baby down safely in a cot for a few minutes to collect themselves is not harmful and is far better than handling a baby while overwhelmed. Those two facts are not in conflict, though the internet often presents them as opposing camps. If crying is relentless and you are at the end of your capacity, put the baby somewhere safe, step away, and call someone. Never shake a baby — that is the one absolute.