From 1993 to 1995, Nagashima was a visiting scholar at Vanderbilt University, Nashville, Tennessee, before becoming a research associate in Asian Security Studies in 1997, and an Adjunct Senior Fellow in Asia Studies at the Council on Foreign Relations, Washington, D.C., in 1999.

From 2000 to 2001, he was a visiting scholar at the Edwin O. Reischauer Center for East Asian Studies at the Johns Hopkins University School of Advanced International Studies (SAIS), Washington, D.C. After coming back to Japan, he taught as a lecturer at Keio University's Graduate School of Law from 2003 to 2007.

Nagashima left the DP in April 2017 due to a disagreement with the party's cooperation with the JCP.

[6] Formerly affiliated to the openly negationist lobby Nippon Kaigi, Nagashima contributed, with Yoshiko Sakurai, Eriko Sanya, and Masahiro Akiyama, to a forum on the Constitution about security, independence, and the article 9 in their journal in July 2009.

[8] In a joint letter initiated by Norbert Röttgen and Anthony Gonzalez ahead of the 47th G7 summit in 2021, Nagashima joined some 70 legislators from Europe and the US in calling upon their leaders to take a tough stance on China and to "avoid becoming dependent" on the country for technology including artificial intelligence and 5G.