This talk shows how to use Python and modern football data to rigorously validate tactical principles instead of merely describing them. Focusing on Germany under Julian Nagelsmann, we will ingest, clean, and structure match data from Hudl Statsbomb and Sportmonks to build a practical tactical analysis pipeline. Using targeted metrics and custom Python workflows, we will test how well performances align with key principles such as gegenpressing intensity, attacking aggressiveness, and defensive compactness across phases of play. Attendees will see how to move from qualitative impressions to quantitative evidence by defining hypotheses, engineering features, and visualizing tactical behaviours over time, ideal for those who know the basics of football data access and now want to support or challenge tactical narratives with hard data.