[It is] the promise of Allah. Allah does not fail in His (30:6)

﴿وَعْدَ اللَّهِ ۖ لَا يُخْلِفُ اللَّهُ وَعْدَهُ وَلَٰكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ﴾
[ الروم: 6]

30:6 [It is] the promise of Allah. Allah does not fail in His promise, but most of the people do not know.

This victory was a promise from Allah, and with its fulfilment, the believers will increase in their conviction of Allah’s promise of victory being true.
As for the majority of people, they do not understand this due to their disbelief.

(It is) a Promise of Allah (i.e. Allah will give victory to the Romans against the Persians), and Allah fails not in His Promise, but most of men know not.

phonetic Transliteration

WaAAda Allahi la yukhlifu Allahu waAAdahu walakinna akthara alnnasi la yaAAlamoona

Abdullah Yusuf Ali - Translation

(It is) the promise of Allah. Never does Allah depart from His promise: but most men understand not.

Safi-ur-Rahman al-Mubarakpuri

A promise from Allah, and Allah fails not in His promise, but most men know not.

Page 405 English transliteration