Russia closed the important Nord Stream 1 export pipeline into Germany three weeks ago, saying Western sanctions were hindering operations. European politicians say this is a pretext by Moscow for weaponising gas supplies.
Here is how Uniper ended up being nationalised.
Under Wednesday's nationalisation plan, Germany will hold 99% of Uniper once it completes a capital increase and buys the holding of its Finnish parent company Fortum.
Uniper relied on long-term supply contracts with Gazprom along with sector peers SEFE - formerly known as Gazprom Germania - and EnBW subsidiary VNG. All three may benefit from a planned gas levy from Oct. 1.
Uniper is weighing legal proceedings against Gazprom in a Swedish arbitration court.
Germany sets the gas levy that would allow Uniper and its rivals to pass on 90% of spiralling gas prices to customers.
The German government agrees to a 15 billion euro Uniper rescue deal and to more than quadruple a credit line with state-lender KfW.
Germany commits itself to raising money via the gas levy, to fund bailouts and uphold gas distribution.
Uniper withdraws 2022 outlook and calls for a bailout.
Uniper implements Moscow's demand to pay for gas in roubles which the European Commission said could breach sanctions.
Uniper hit by a 3 billion euro first quarter net loss, posts impairments related to its Russian generation unit.
Uniper tries to appease investor concerns by signalling exit from Russia and sale of its 83.7% Unipro stake.
Uniper says it will propose a 95% cut in its 2021 dividend.
The Nord Stream 2 pipeline link in which it was a co-financier, is abandoned amid Russia's invasion of Ukraine, leaving a 1 billion euro write-down. NS 2 would have doubled the NS 1's capacity of 55 billion cubic metres a year.
Russia on Feb. 24 invades Ukraine, calling it a "special military operation" to disarm its neighbour.
Uniper secures credit facilities worth 10 billion euros from main shareholder Fortum and German state bank KfW plus 1.8 billion euros in bank loans in a precautionary move to address high prices and volatility.
Global gas markets became tight in a post-COVID recovery and Russia started sending less westwards, pointing to the imminent start of the Nord Stream 2 pipeline.
Uniper, which was already feeling the strain from a decarbonisation shift that forced the shutdown of coal plants, maintained throughout that Russia was a reliable shipper.
Uniper represents the legacy business of Ruhrgas which E.ON took over in 2003 and fully absorbed in 2013.
The purchase which cemented joint German-Russian trade and hydrocarbon exploration links was opposed by the cartel office and by some as pandering to pro-Moscow business interests.
E.ON spun off a majority of Uniper in 2016 and later agreed to sell its remaining stake to Fortum.
($1 = 1.0087 euros)
(Reporting by Vera Eckert, editing by Miranda Murray and Alexander Smith)