Diligence gets the scrutiny, but the deals that underperform their thesis almost always trace back to the same window: the first hundred days after close, when integration decisions get made under time pressure by teams still learning each other's shorthand.
Decide the operating model before day one
Waiting until after close to decide how the combined organization will run wastes the period when employees are most receptive to change. The target operating model should be scoped during diligence, not designed after signing.
Protect the synergy case with a named owner per line item
Synergy targets that live only in a spreadsheet rarely survive contact with reality. Every line item needs an accountable owner and a monthly tracking cadence from week one, not a quarterly retrospective.
Momentum is a resource, not a metaphor
Employees, customers, and suppliers give a newly combined company a limited window of goodwill to sort itself out. Spend it deliberately on the few changes that matter most, rather than diffusing it across every workstream at once.