Was the Zimmermann Telegram so self-defeating that it was probably a British conspiracy?
Short answerNo; it fits a genuine pattern of contradictory German decisions that alienated the United States.
NotesThe Zimmermann Telegram need not have been a British fabrication simply because it was strategically foolish. Germany also depended on American access for low-volume but important war-industry supplies. German submarines were meanwhile operating near American waters and attacking merchant shipping in ways that embarrassed the United States. A government can pursue policies that undermine one another without requiring a hidden conspiracy.