the critical moment, the moment of truth
Ele ia enfrentar o chefe sobre um aumento, mas na hora H, acovardou-se.
He was going to confront his boss about a raise, but at the moment of truth, he chickened out.
The expression is military and dates to World War I, not World War II. The U.S. Army borrowed the practice from the French: in operation orders the day an attack would begin was marked D-Day ("Dia D") and the exact start time H-Hour ("Hora H"), with the letters simply standing for "day" and "hour" so plans could be drawn up before the real date was fixed or kept secret.
Source: Wordorigins.org.
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