Summary: The rules by which we are to conduct ourselves as strangers in this world are provided in the Word of God.
“Humble yourselves, therefore, under the mighty hand of God so that at the proper time he may exalt you, casting all your anxieties on him, because he cares for you. Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour. Resist him, firm in your faith, knowing that the same kinds of suffering are being experienced by your brotherhood throughout the world. And after you have suffered a little while, the God of all grace, who has called you to his eternal glory in Christ, will himself restore, confirm, strengthen, and establish you. To him be the dominion forever and ever. Amen.” 
Civilised nations have drafted rules to guide their conduct, even in war. In the aftermath of World War II, the first three treaties previously drafted in 1864, 1906 and 1929, were updated. Nations bound themselves to abide by these protocols of war and further agreed to a set of rules for humanitarian treatment of prisoners of war, for treatment of the wounded and for handling civilian non-combatants. These rules are commonly known as the Geneva Conventions of War. Modern wars, often referred to as limited engagements, tend to disregard these conventions, though those warring against the western powers demand that their own people be treated in accordance with these protocols.
Christians are engaged in a protracted war with the ruler of this world. Those dark powers aligned with the evil one are merciless and without pity. They disregard all pleas from the wounded or those caught up in the war. However, we who are identified with the King of kings and the Lord of lords are bound by His Word to conduct ourselves according to the rules of engagement that He has given us. It will be to our benefit to know His will and to ensure that we do His will. These, then, are the rules of engagement given by the Son of God.
CONTEXT —Believers are engaged in a conflict; this is not a brief conflict—it began before the fall of our first parents and has continued unabated since that time. A powerful angel, once known as Day Star [see ISAIAH 14:12], rebelled against the True and Living God. Because of His beauty, he grew proud and determined in his heart to displace the Lord God from His eternal throne [see ISAIAH 14:13, 14]. He was cast out of heaven and to the earth [see EZEKIEL 28:12-19]; there, he was instrumental in deceiving our first mother and plunging the world into its present condition of sin and ruin. He is set in opposition to God and to all who seek to serve the Lord God of Heaven and earth. He is the enemy of the soul, slandering the saints before the Most High [cf. JOB 1:6-12; 2:1-7]. For the moment, he functions as “the ruler of this world” [JOHN 12:31] and “the god of this world” [2 CORINTHIANS 4:4], though his destiny is destruction when he shall at last be cast into the lake of fire [REVELATION 20:9, 10]. The American President may fight wars on a timetable; but this is a war that must be fought until victory is achieved.