The Quran

Preface

فَإِذَا انسَلَخَ الأَشهُرُ الحُرُمُ فَاقتُلُوا المُشرِكينَ حَيثُ وَجَدتُموهُم وَخُذوهُم وَاحصُروهُم وَاقعُدوا لَهُم كُلَّ مَرصَدٍ ۚ فَإِن تابوا وَأَقامُوا الصَّلاةَ وَآتَوُا الزَّكاةَ فَخَلّوا سَبيلَهُم ۚ إِنَّ اللَّهَ غَفورٌ رَحيمٌ

Yusuf Ali

But when the forbidden months¹ are past, then fight and slay² the Pagans wherever ye find them, an seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent,³ and establish regular prayers and practise regular charity, then open the way for them: for God is Oft-forgiving, Most Merciful.

The emphasis is on the first clause: it is only when the four months of grace are past, and the other party shows no signs of desisting from its treacherous designs by right conduct, that the state of war supervenes - between Faith and Unfaith.
When war becomes inevitable, it must be prosecuted with vigour. According to the English phrase, you cannot fight with kid gloves. The fighting may take the form of slaughter, or capture, or siege, or ambush and other stratagems. But even then there is room for repentance and amendment on the part of the guilty party, and if that takes place, our duty is forgiveness and the establishment of peace.
The repentance must be sincere, and that is shown by conduct - a religious spirit of true prayer and charity. In that case we are not to bar the gate against the repentant. On the contrary we must do all we can to make their way easy, remembering that God is Oft-Forgiving, Most Merciful.