1 - why I suppose to have any position about Islam? Islam for Muslims.
2 - how you can explain this - فَإِذَا انسَلَخَ الأَشْهُرُ الْحُرُمُ فَاقْتُلُواْ الْمُشْرِكِينَ حَيْثُ وَجَدتُّمُوهُمْ وَخُذُوهُمْ وَاحْصُرُوهُمْ وَاقْعُدُواْ لَهُمْ كُلَّ مَرْصَدٍ فَإِن تَابُواْ وَأَقَامُواْ الصَّلاَةَ وَآتَوُاْ الزَّكَاةَ فَخَلُّواْ سَبِيلَهُمْ إِنَّ اللّهَ غَفُورٌ رَّحِيمٌ
surah 9:5
And when the sacred months have passed, then kill the polytheists wherever you find them and capture them and besiege them and sit in wait for them at every place of ambush. But if they should repent, establish prayer, and give zakah, let them [go] on their way. Indeed, Allah is Forgiving and Merciful.
3 - why?
This particular verse you quoted is talking about a certain group of disbelievers who made a treaty with the prophet peace be upon him but broke it. You can see that from the start of the chapter.
1 [This is a declaration of] disassociation,
from Allah and His Messenger, to those with whom you had made a treaty among the polytheists.
2 So travel freely, [O disbelievers], throughout the land [during] four months but know that you cannot cause failure to Allah and that Allah will disgrace the disbelievers.
3 And [it is] an announcement from Allah and His Messenger to the people on the day of the greater pilgrimage that Allah is disassociated from the disbelievers, and [so is] His Messenger. So if you repent, that is best for you; but if you turn away - then know that you will not cause failure to Allah . And give tidings to those who disbelieve of a painful punishment.
4 Excepted are those with whom you made a treaty among the polytheists and then they have not been deficient toward you in anything or supported anyone against you; so complete for them their treaty until their term [has ended]. Indeed, Allah loves the righteous [who fear Him].
5 And when the sacred months have passed, then kill the polytheists wherever you find them and capture them and besiege them and sit in wait for them at every place of ambush.
But if they should repent, establish prayer, and give zakah, let them [go] on their way. Indeed, Allah is Forgiving and Merciful.
6
And if any one of the polytheists seeks your protection, then grant him protection so that he may hear the words of Allah . Then deliver him to his place of safety. That is because they are a people who do not know.
7
How can there be for the polytheists a treaty in the sight of Allah and with His Messenger, except for those with whom you made a treaty at al-Masjid al-Haram? So as long as they are upright toward you, be upright toward them. Indeed, Allah loves the righteous [who fear Him].
8 How [can there be a treaty] while, if they gain dominance over you, they do not observe concerning you any pact of kinship or covenant of protection? They satisfy you with their mouths, but their hearts refuse [compliance], and most of them are defiantly disobedient.
However, as for combating, the rules in Islam is clear through other verses.
Islam is a system of life which tells you how you are supposed to live your life. So it provides answers or a pattern of thinking/response to every single situation you would face. If verses were removed, it will cease to be perfect and it would open people to retaliate the way they want and not the way God wants them too. Since Islam is a complete way of life it is normal to find verses where it tells you how to react when you are facing a war because regardless of whether we like that or not, it is part of life.
2:190 Fight in the way of Allah those who fight you but do not transgress. Indeed. Allah does not like transgressors.
2:191 And kill them wherever you overtake them and expel them from wherever they have expelled you, and fitnah is worse than killing. And do not fight them at al-Masjid al- Haram until they fight you there. But if they fight you, then kill them. Such is the recompense of the disbelievers.
2:192 And if they cease, then indeed, Allah is Forgiving and Merciful.
2:193
Fight them until there is no [more] fitnah and [until] worship is [acknowledged to be] for Allah . But if they cease, then there is to be no aggression except against the oppressors.
2:194
[Fighting in] the sacred month is for [aggression committed in] the sacred month, and for [all] violations is legal retribution. So whoever has assaulted you, then assault him in the same way that he has assaulted you. And fear Allah and know that Allah is with those who fear Him.
If you look at the conditions to fight those than you will find,
1- They are the ones who started the fight.
2- They transgressed,.
3- Third they are the ones who expelled you out of your home.
4- regardless of all that, if they stopped, you have to stop too.
I don't think there exist anyone who has these ethics when it comes to war.