Word by Word Analysis

فَإِذَا جَآءَ وَعْدُ أُولَىٰهُمَا بَعَثْنَا عَلَيْكُمْ عِبَادًا لَّنَآ أُو۟لِى بَأْسٍ شَدِيدٍ فَجَاسُوا۟ خِلَـٰلَ ٱلدِّيَارِ ۚ وَكَانَ وَعْدًا مَّفْعُولًا 5

So when the [time of] promise came for the first of them, We sent against you servants of Ours - those of great military might, and they probed [even] into the homes, and it was a promise fulfilled.