Without observance of Salah and Zakah, no one can attain the status of 'muslim' ('believer') in the Islamic Community!

Verse 11 of Chapter 9:

But if they repent, perform the Salah and give the Zakah, then they are your brethren in religion. (In this way) We explain the Ayat in detail for a people who know.

What is the common principle  does God apply to withdraw the favor He bestows on any people?

Verses 53-54 of Chapter 8: 

This happened in accordance with Allah's way that He does not withdraw the favor He bestows on any people unless they themselves change their ways. Allah hears everything and knows everything. This same principle was applied to the people of Pharaoh and the other peoples before them: when they treated the revelations of Allah as false, We destroyed them in consequence of their sins, and drowned the people of Pharaoh; all these peoples were the workers of iniquity.

What strategy did God design and implement to start the 1st war between the believers under the leadership of the last prophet and the disbelievers in order to reveal the truth?

 

Verses 43-44 of Chapter 8:

And remember) when you (the Muslim army) were on the near side of the valley, and they on the farther side, and the caravan on the ground lower than you. Even if you had made a mutual appointment to meet, you would certainly have failed in the appointment, but (you met) that Allah might accomplish a matter already ordained (in His knowledge), so that those who were to be destroyed (for rejecting the faith) might be destroyed after a clear evidence, and those who were to live (believers) might live after a clear evidence. And surely, Allah is All-Hearer, All-Knower. And remember) when Allah showed them to you as few in your dream; if He had shown them to you as many, you would surely have been discouraged, and you would surely have disputed in making a decision. But Allah saved (you). Certainly, He is the All-Knower of that is in the breasts. And (remember) when you met, He showed them to you as few in your eyes and He made you appear as few in their eyes, so that Allah might accomplish a matter already ordained, and to Allah return all matters (for decision).