Expansion franchise Philadelphia Union have acquired the rights to USA national team defender Danny Califf.

The 29-year old will return to MLS after four years in Denmark with Aalborg BK and FC Midtjylland and is a huge addition to Peter Nowak's playing roster for the Union's debut season.

"There will be competition for every spot on the pitch," Nowak said in a press release. "But with a seasoned veteran like Danny Califf, we're adding a player who will help us set the bar ever higher.

"Danny understands the level of commitment our technical staff will demand daily and we're counting on him to not only help anchor our defense but push our younger players to greater heights, too."

Califf has been capped 23 times at international level and also has MLS experience after winning the 2002 MLS Cup with the LA Galaxy before joining the San Jose Earthquakes prior to his European adventure.