When Honda reformed its alliance with McLaren ahead of the 2015 Formula 1 season, an agreement was made whereby both parties had to approve if the Japanese manufacturer would supply a second team.
Initially, there was no real interest from either side to do so given Honda's struggles for performance and reliability in the first season. McLaren wanted to be the sole focus while Honda got on top of its problems, and Honda didn't want to consider stretching itself until it had its foundations in place.
When Red Bull needed an engine for 2016, having fallen out with Renault, then-McLaren Technology Group CEO Ron Dennis blocked a potential deal for Honda to step in. He said McLaren didn't want to put more pressure on the supply system and make Honda's problems worse.