Hägglund commanded United Nations troops in 1978–1979 as the commander of the Finnish battalion (FINBATT) in UNEF II in Sinai, as the commander of UNDOF in Golan in 1985–1986, and as the commander of UNIFIL in Southern Lebanon in 1986–88.[2][3]
Hägglund served under Presidents Martti Ahtisaari and briefly under Tarja Halonen. Hägglund's command of the defence forces was characterised by the general modernisation of the force and refocusing from quantity to quality. In the Army, three large "readiness brigades" were formed, and the Air Force upgraded to Hornet F/A-18 fighters. Finland entered NATO's Partnership for Peace programme under Hägglund's command.
A major change was made in terms of conscription service: reserve officers and NCOs would serve 12 months (lengthened from 11 months), but rank and file would serve 6 months (shortened from 8 months), with some specialists such as military police serving 9 months. Previously the rotation had been thrice per year, in order to maintain a contingent that had been trained at least 4 months. The rotation was changed to twice per year.
Within the military, Hägglund was also a strong proponent of peacekeeping operations, worked to improve their appreciation and opined that officers should necessarily participate. Since Finnish law requires that all participants be volunteers, this brought him into conflict with the officers' union, whose opinion was that peacekeeping experience could not be a requirement for career advancement.
Officer training went through a major improvement due to his actions. The cadet school was promoted to university status as the Finnish National Defence University, and degrees were formalised.
Hägglund has published three books, one concerning the defence of Finland, one about the defence of Europe and an autobiography. In the 2000s, Hägglund promoted the development of the independent defence of Europe, and took the view that the United States would shift its focus away from Europe, changing the role of NATO.
^"Itse asiassa kuultuna: Kenraali Gustav Hägglund" (in Finnish). Yle. 3 December 2018. Retrieved 5 August 2020. YK-tehtävissä hän on ollut kolmessa paikassa: YK:n suomalaisen valvontajoukon komentajana Siinailla, Golanilla Undof-joukkojen komentajana ja Unifil-joukkojen komentaja Libanonissa.