وَأَمَّا مَنْ ءَامَنَ وَعَمِلَ صَٰلِحًا فَلَهُ جَزَآءً ٱلْحُسْنَىٰ ۖ وَسَنَقُولُ لَهُ مِنْ أَمْرِنَا يُسْرًا

Dhu’l-Qarnayn, in making his conquests, possibly proceeded in a westward direction from Iran, and thus reached Asia Minor, halting only when he came to the ‘black water’ of the Aegean Sea. If one stands on the sea-shore here and looks out over the sea in the evening, it will appear as if the sphere of the sun is sinking into the water and setting there. This is a figurative description of the furthermost point which Dhu’l-Qarnayn reached. Dhu’l-Qarnayn had reached this seashore not as a wayfarer but as a conqueror. Thus he gained full control over the people inhabiting that area and established his rule there. As a ruler he had full powers to deal with the inhabitants as he liked. But Dhu’l-Qarnayn was a just king. He was not cruel to anybody. He made the general announcement that he would deal severely only with anyone found indulging in evil deeds and that no excess would be perpetrated upon those who maintained peace and order.

— Source: Tazkirul Quran (Maulana Wahiduddin Khan) via Quran.com (18:88)