April 22 (Reuters) - Deutsche Telekom DTEGn.DE is exploring a deal to combine with its U.S. unit, T-Mobile US TMUS.O, according to sources, in what would be a record-breaking public merger.

The deal would crown a quarter century of maneuvering that transformed an American upstart into a $218 billion juggernaut and made the U.S. business the crown jewel of its German parent.

Here are milestones in the relationship between Deutsche Telekom and T-Mobile:

1999 - Western Wireless spins off its VoiceStream Wireless unit into the VoiceStream Wireless Corporation.

2000 - Deutsche Telekom agrees to acquire VoiceStream Wireless and U.S. GSM operator Powertel. The deal is valued at approximately $50.7 billion at the time of the announcement.

2001 - Deutsche Telekom completes its acquisitions of VoiceStream and Powertel, forming what it calls the world's first transatlantic wireless communications operator using the GSM standard. VoiceStream becomes part of T-Mobile International, Deutsche Telekom's mobile subsidiary.

2002 - VoiceStream, rebranded as T-Mobile USA, begins rolling out the T-Mobile brand in major western U.S. cities, including Las Vegas, Los Angeles and San Francisco.

2011 - AT&T T.N announces plans to purchase T-Mobile US for $39 billion, but the deal collapses after the intervention by U.S. antitrust regulators. Deutsche Telekom receives $4 billion in breakup compensation.

2013 - T-Mobile USA merges with MetroPCS. The combined company begins trading under the name T-Mobile US and the ticker symbol TMUS on the New York Stock Exchange on May 1.

2020 - T-Mobile US completes an all-stock merger with Sprint valued at $26 billion. Deutsche Telekom's stake in T-Mobile US shrinks to 43%, while Sprint's owner, Japan's SoftBank 9434.T, takes a 24% stake.

2021 - Deutsche Telekom sells T-Mobile Netherlands for 5.1 billion euros, or $6 billion in today's exchange rate, and uses some of the proceeds to buy back T-Mobile US shares.

2022 - Deutsche Telekom acquires 21.2 million T-Mobile US shares from SoftBank for $2.4 billion, increasing its stake to 48.4%.

2023 - Deutsche Telekom surpasses a 50% stake in T-Mobile US. During a Deutsche Telekom general meeting, CEO Tim Hoettges says that the company had been pursuing a majority stake since 2021.

2026 - Deutsche Telekom is exploring a deal to combine with T-Mobile US to create a $400 billion telecoms giant in what would be the largest-ever public merger, two people familiar with the matter say.

($1 = 0.8515 euros)