The invitation to Latvia was extended based on the Vienna Document, which is an agreement between the participating states of the Organization for Security and Cooperation in Europe (OSCE) about confidence and security building measures, including an annual exchange of military information about forces located in Europe, prior notification of certain military activities and their observation, inspection and evaluation visits etc.
Both Lithuania and Estonia have already received invitations to observe the large-scale Belarusian-Russian ZAPAD (West) drills. The Latvian response to the invitation was not specified, but speaking in Riga in June, NATO Secretary General Jens Stoltenberg said he hoped such an invitation would be forthcoming. Asked about Zapad 2017 and said:
"We don’t see any imminent threat against any NATO allied country, but what we have seen is a more assertive Russia, we have seen a significant military buildup also in the Baltic region and we have seen Russia being willing to use military force against neighbours as we have seen in Georgia and in Ukraine... We’ll of course follow and monitor closely the Russian exercise ZAPAD.
"We call on Russia to be transparent related to the ZAPAD exercise because that’s part of what we seek in our dialogue with Russia: transparency, predictability and measures to achieve risk reduction. And of course, it is important to misunderstand [sic] … or to avoid any misunderstanding, to avoid any miscalculations and therefore we are now looking into the possibility of holding a meeting of the NATO Russia Council where we’ll ask or call on Russia to brief on the ZAPAD exercise and at the same time we are calling on Russia to be transparent on the ZAPAD exercise in line with the Vienna Document which requires transparency, international observation of these kinds of exercises."
Meanwhile ZAPAD is the focus of intense activity among Russia-watchers.