Two years after a congressional report labeled Huawei Technologies Co. a national-security threat, the Chinese firm unexpectedly scored a big-name ally in Washington.

It was the Redskins, the capital’s National Football League franchise. Huawei 002502, -1.44% reached an agreement in 2014 to beam Wi-Fi through the suites at the team’s FedEx Field, in exchange for advertising in the stadium and during broadcasts.

It was a marketing coup for a company hankering to beef up its meager U.S. business and boost its image inside the Beltway. But the deal didn’t last long.

A government adviser read about the partnership. He knew the FedEx FDX, +1.16% Field suites were a frequent haunt for lawmakers and senior officials across many agencies. So he triggered an unofficial federal complaint to the Redskins, who quietly tore up the deal.

That previously unreported backroom maneuver is an example of a yearslong effort by U.S. officials, often working outside formal channels, to blacklist the Chinese technology giant. Washington has since intensified the campaign and taken it mainstream, with Congress and federal agencies working this year to snuff out Huawei’s small U.S. business and curtail its much bigger overseas ambition.

An expanded version of this report appears on WSJ.com.

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