Use 'perforce' to imply something must occur out of necessity, without any alternative options. The bridge was out, and we had to take the longer route, perforce.
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Formal Tone
'Perforce' is considered formal and is more commonly found in written English than spoken language. She perforce accepted the terms of the agreement, having no leverage to negotiate.
Pro Tip 3/3
Historical Flavor
Using 'perforce' can give your language an old-fashioned or historical feel, as it's not common in modern casual speech. The knights, perforce, held the castle against all odds.