The US government's involvement with geo-engineering goes all the way back to Project Stormfury (https://en.wikipedia.org/wiki/Project_Stormfury). Nowadays their most prominent effort (debatable) is through ionosphere heating via ground stations (such as HAARP). They used to be more adamant about solar radiation management but have for w/e reason cut down on this effort since 2015. This chem-trail explosion began in earnest in the mid 90s with additives added to aviation fuel which were dispersed by commercial (non-military) and military aircraft. As well, the growth of aviation travel increased chem-trail coverage.
The canary in the geoengineering coal mine is of course the global warming hiatus and increased ocean mixing since 2000-2002 or so. The rate of surface temperature increase over land regions had abruptly slowed down. There are two caveats around the geoengineering problem that make it easy to dismiss.
1.) The slow-down in surface temperature warming came after the largest el nino event of the 20th century. As a result a la nina developed and cooling ensued.
2.) Lower surface temperatures as a result of aerosol emissions are a side-effect rather than a conscious effort to cool the planet. (This seems more likely as aviation fuel is much cleaner and burns more efficiently than it did in the 1990s).
The official publicly known purpose of HAARP as a military/scientific facility is a missile defense system and/or a space defense system. It is possible as well that in the process of conducting other activities, the facility alters climate unintentionally but the effect seems minimal to date. The true forcing of all greenhouse gases emitted by human sources thus remains blunted by the negative forcing of aerosols. Intentional or not, the end result is the same. However I believe the aerosol forcing is weaker than it was two decades ago, owing to improved air quality in most regions. Especially Europe and the US.