Shaikh-ul-Islam Ibn Taymiyyah (Rahimahullah) said:
وَقَدْ ذَهَبَ كَثِيرٌ مِنْ مُبْتَدِعَةِ الْمُسْلِمِينَ: مِنْ الرَّافِضَةِ وَالْجَهْمِيَّة وَغَيْرِهِمْ إلَى بِلَادِ الْكُفَّارِ فَأَسْلَمَ عَلَى يَدَيْهِ خَلْقٌ كَثِيرٌ وَانْتَفَعُوا بِذَلِكَ وَصَارُوا مُسْلِمِينَ مُبْتَدِعِينَ وَهُوَ خَيْرٌ مِنْ أَنْ يَكُونُوا كُفَّارًا وَكَذَلِكَ بَعْضُ الْمُلُوكِ قَدْ يَغْزُو غَزْوًا يَظْلِمُ فِيهِ الْمُسْلِمِينَ وَالْكُفَّارَ وَيَكُونُ آثِمًا بِذَلِكَ وَمَعَ هَذَا فَيَحْصُلُ بِهِ نَفْعُ خَلْقٍ كَثِيرٍ كَانُوا كُفَّارًا فَصَارُوا مُسْلِمِينَ وَذَاكَ كَانَ شَرًّا بِالنِّسْبَةِ إلَى الْقَائِمِ بِالْوَاجِبِ وَأَمَّا بِالنِّسْبَةِ إلَى الْكُفَّارِ فَهُوَ خَيْرٌ.
“And many of the Muslim innovators, from the Rafidah and the Jahmiyyah, and others, went to the lands of disbelievers. Many people embraced Islam at their hands, benefiting from them, and became Muslim-innovators, which is better for them than remaining disbelievers. And similarly, some of the Kings (Muslim rulers) may engage in unjust wars, harming both Muslims and disbelievers, and they may be sinful in doing so. Nevertheless, good comes from it for many people who were disbelievers and (they then) became Muslims. This is considered evil in relation to the one responsible for fulfilling the obligation, but it is good in relation to the disbelievers.”
Majmua-Fatawa Ibn Taymiyyah (13/96).
(Translated by Mohammed Manna.)
Saheehus-Seerah 