According to the Armaverse Timeline, the events in ARMA 2 (2009) concluded before the Outbreak and the events portrayed in DayZ, which started between August 2012 and the "offfical" release of February 2013.
However, there remains consistent speculation about which factions were actually involved in DayZ lore, taking the in-game assets into account from both the Mod and Standalone and I've read some pretty detailed (and in some cases outlandish) lore on this forum to this effect. Here's the complete list as I currently understand it (with a brief from my viewpoint). Please add your ideas, thoughts and justifications.
Army of the Czech Republic: due to Sa.58 in the Mod. However, this could simply be foreign weapons imports, which are common during a civil war like in ARMA 2.
British Armed Forces: due to DPM skin, Lee Enfield, L85A2 and FN FAL in the Mod, and UK Assault vests and DPM camo items plus FN FAL in Standalone. Again, this could simply be foreign military equipment imports during a civil war.
Bundeswehr: due to G36 in Mod and various Flecktarn items in Standalone. This has been used as evidence to "prove" that the KSK was oper8ing in Chernarus. Then again, NAPA was known to wear Flecktarn during 2009 and the weapons could be imports.
Chernarus Defence Forces: berets, TTsKO camo assets plus weapons. These guys were clearly there before and during the Outbreak.
Chernarussian Movement of the Red Star: beret asset. Assumed to be present due to ARMA 2 history.
ION Private Military Contractors: seen some speculations that some of these guys may have been involved.
New Zealand: due to the single beret asset. This is likely just an easter egg due to Rocket's prior service there.
Russians: assumed to present due to ARMA 2 campaign. Perhaps as "peacekeepers" just prior to DayZ?
UN: due to the helmets and berets as well as the Balota base and various aid stations and roadblocks. There seems to have been a UN presence in South Zagoria prior to DayZ, which implies that if you take the Operation Harvest Red campaign as canon, the outcome was either Setting Sail or The War That Never Was. These UN peacekeepers therefore could have Russian and/or these UN peacekeepers could also have been there specifically to deal with the beginnings of the Outbreak.
United States Army: seen some speculations that some of these guys may have been involved.
United States Marine Corps: due to M14 and M16A2 in the Mod and MARPAT and BDU clothing in Standalone. USMC has long been assumed to be involved in the immediate background of DayZ due to the assets and their prior involvement in the ARMA 2 campaign.