At least one Israel Defense Forces soldier fighting in Gaza is the son of dear friends of my family. I want to focus my tefilla on his welfare, by name, although I certainly pray for the well-being of every Israeli soldier and civilian. Which tehillim are traditionally read for this purpose, and why were those psalms selected?
While I am not aware that there is any authoritative list, It seems that among several that are coomonly said is Tehillim 121.
I believe this was chosen as a general Tehillim to be said for people who are ill as well as people in danger, such as soldiers. The reason is because it starts with the phrase
"I lift my eyes to the mountains, from where will my help come? My help comes G-d, maker of heaven and earth..."
The rest of this chapter of Tehillim contains verses of hope and the general concept that God will watch you and protect you from all danger and evil.
Based on what I've seen, Psalm 130 is also commonly recited with 121.
Additionally, it would appear to me that Psalm 20 is appropriate based on its references to HaShem fighting for us.
In the Metsudah Tehillem and http://www.tehilimhotline.org/prayer_categories.asp it says which chapters should be said at which occaison, the one's pertaining to this matter would be(I think):
For the Jewish People 43, 79, 80, 83
For help in troublesome times 20, 38, 85, 86, 102, 130, 142
For peace 46
For success 112
Chabad.org (http://www.chabad.org/library/article_cdo/aid/809299/jewish/Psalms-for-Times-of-Distress.htm ) says also: 20, 22, 69 150
So all the good Prakim would be: 20, 22, 38, 43, 46, 69, 80, 83, 85, 86, 102, 112, 130 142, 150.