From 2002, Arnold was part of the parliamentary group's leadership under successive chairmen Franz Müntefering, Peter Struck, Frank-Walter Steinmeier, and Thomas Oppermann.

[2] In his capacity as member of the Defence Committee, Arnold traveled extensively to visit Bundeswehr troops on their missions abroad, including Kosovo (2002, 2003, 2004, 2006, 2012), Djibouti (2006), Democratic Republic of the Congo (2006), Afghanistan (2007, 2008, 2009, 2010, 2011, 2012, 2014) and Mali (2013, 2014).

In January 2015, he accompanied German Minister of Defence Ursula von der Leyen on a visit to Jordan, where they met with King Abdullah II, among others.

[5] Arnold has in the past voted in favor of German participation in United Nations peacekeeping missions as well as in United Nations-mandated European Union peacekeeping missions on the African continent, such as in Somalia – both Operation Atalanta (2010, 2011, 2014 and 2015) and EUTM Somalia (2014, 2015 and 2016) –, Darfur/Sudan (2010, 2011, 2012, 2013, 2014, 2015 and 2016), South Sudan (2011, 2012, 2013, 2014, 2015 and 2016), Mali – EUTM Mali (2013, 2014 and 2015) and AFISMA (2013), and MINUSMA (2013 and 2014) –, the Central African Republic (2014) and Liberia (2015).

And sometimes they committed crimes.” He held that the breadth of Germany's social safety net, with its inexpensive health care, relatively generous unemployment benefits and open access to higher education, means that fewer services are needed specifically for soldiers.