Why the crucifix?

Although I’ve tried off and on since college, I’ve really made a concerted effort in the past few months to pray Morning and Evening Prayer from the Liturgy of the Hours every day. So far I haven’t missed any, but I’ll admit, sometimes Morning Prayer gets delayed until 5 pm.
Anyway, a prayer that should be fairly familiar to everyone who does pray the Office daily is Psalm 149. Featured as the third psalm/canticle in Morning Prayer of Sunday, Week I, Psalm 149 is said, not only every 4th Sunday, but also on every feast and solemnity.
The Psalmist begins by calling the Israelites to rejoice in their God with great joy and merriment. “Let them praise his name with dancing and make music with timbrel and harp” (Ps 149:3). Towards the end, however, his tone changes to a battle hymn of sorts.
Let the praise of God be on their lips
and a two-edged sword in their hand,
to deal out vengeance to the nations
and punishment on all the peoples;
to bind their kings in chains
and their nobles in fetters of iron;
to carry out the sentence pre-ordained;
this honor is for all his faithful. (Ps 149:6-9)
You might ask yourself “What is going on here?” How did we go from “Sing a New Song Unto the Lord” to “Onward, Christian Soldiers” in just a few verses? Even more so, why does the Church have her priests, deacons, religious, and lay faithful throughout the world recite this psalm on a regular basis?
Some might say that the Psalms were composed during the Kingdom of Israel days when there was a physical army waging physical wars against people who sought to physically force the Hebrews to abandon their worship of the One, True, God. This is definitely a part of the story.
We can also regard the Kingdom of Israel as a type (or foreshadowing image) of Christ's Kingdom. The Kingdom of Israel was earthly, Christ's is heavenly; Israel was temporal, Christ's is eternal; etc. When we read the Psalms and their talk of kingdoms and subjugation of foes, we shouldn’t just poo poo it as an artifact of a bygone age. We know that what applied to the Kingdom of Israel applies even more so to Christ's Kingdom, and we should know that any kingdom has enemies. In keeping with the supernatural nature of Christ’s Kingdom, though, we should expect supernatural enemies.
Should our battle against the supernatural foes of the New Jerusalem be any less important to God than the battles of Jerusalem against the physical foes of the Israelites? Would we be abandoned today by a God who once granted a little shepherd boy from backwater Israel victory over a highly trained warrior giant from Philistia?
The answer to both questions, of course, is “No.”
And, yes, times may seem difficult, and the battle may seem too strongly against our favor. But know that we are ultimately called, not just allowed but called, to conquer sin and Satan; to bind the "Prince of Lies" in chains, and his demons in fetters of iron. For this awesome responsibility, we can do nothing but offer unreserved praise to Our God.