Here is a list of the books on this subject
http://www.isfdb.org/cgi-bin/pe.cgi?12384 * How Few Remain Universe
o How Few Remain (1997) by Harry Turtledove
o The Great War
+ The Great War: American Front (1998) by Harry Turtledove
+ The Great War: Walk in Hell (1999) by Harry Turtledove
+ The Great War: Breakthroughs (2000) by Harry Turtledove
o Settling Accounts
+ 1 Return Engagement (2004) by Harry Turtledove
+ 2 Drive to the East (2005) by Harry Turtledove
+ 3 The Grapple (2006) by Harry Turtledove
+ 4 In at the Death (2007) by Harry Turtledove
o The American Empire
+ 1 Blood and Iron (2001) by Harry Turtledove
+ 2 The Center Cannot Hold (2002) by Harry Turtledove also appeared as:
# Variant Title: The Centre Cannot Hold (2002)
+ 3 The Victorious Opposition (2003) by Harry Turtledove Guns of the South involves time traveling South African giving Lee modern weapons , a good book but not part of the series.