I liked the way Wagner approached this topic, with thoughtfulness, a blend of experience and anecdote, along with some solid theorising. However I am not sure he is right in the end. The central premise is that Christians need to be engaged in spiritual warfare at a strategic level - against powers and principalities over territories and counties. A lot of this makes sense and the book of Daniel along with Ephesian's strongly supports that such do exist.
However if Christians are waking up to the reality of such warfare (and initial results in Argentina to use Wagner's favourite testing zone - did suggest astonishing success in evangelism) then a few decades later surely the fruit should be even more strongly evident?!? However even in the places where initial success was most evident things seemingly have stalled rather than gone on to greater results. Possibly the strategic prayer and spiritual warfare has slackened off and that is the reason, but I wonder if there is more to it?
There is very little - or even no evidence, of biblical strategic spiritual warfare such as Wagner encourages. Rather such things are left to the angels and the New Testament may well suggest this is where is should remain - we are only the foot soldiers in this war. Getting mixed up in so called strategic level spiritual warfare violates the principle of 'pray positive!'. That is, don't pray against something, but for something - bless don't curse, even the enemy.
The general intention is to pray for the gospel to advance not for the devil to retreat! The reason for this is alongside of the principle of the power of praise - whereby we praise God even in times of trouble and this somehow enables things to open up in the spiritual. In contrast praying against the enemy in the long run glorifies his power and dominion, and while it may lead to short term results leads to nothing long term.
However this is not to say there aren't times and places where the Holy Spirit will clearly lead us into such a situation - however be doubly cautious, particularly when faced with an initial flush of success. The one thing we don't want to find ourselves doing is 'praying to the enemy' instead of the Lord, albeit in rebuke et al.
In fairness, while Wagner in the era this book appeared doesn't determine against this kind of spiritual warfare, he does proceed with many cautions attached. I particularly like his six rules for city taking - 1) set a manageable geographic area, 2) secure the unity of the church leaders , 3) make it a whole body of Christ thing (not denominational/sectional), 4) prepare the participants in getting right with God first, 5) research the history, 6) work with recognisable intercessors for the approach.
Overall a worthy book, but don't proceed to far on it's strength alone.
One more question remains unanswered - such spiritual powers are easily identified in developing world countries where historical superstition is rampant, but what of the modernised west? What is the nature of the strategic spiritual opposition here?