SEOUL, Dec 19 (Reuters) - As South Korean President Yoon Suk Yeol's relations with the opposition broke down this year, Western diplomats hoped the internal tensions would not affect his tough stance on China and North Korea that had won praise from Washington.

It didn't work.

A focus on a foreign policy seen as favourable to the West, limited attention on the political discord in South Korea, and a hesitation to be seen interfering in domestic matters, left its partners blindsided when Yoon briefly imposed martial law on Dec. 3, analysts and current and former diplomats said.

Yoon is now impeached and his powers suspended - and his political implosion likely heralds the return of the political left, which is less inclined to be as unabashedly supportive of the U.S. and Japan.

"His allies in Washington are concerned about one thing and one thing only - U.S. national security," said Karl Friedhoff of the Chicago Council on Global Affairs. "When I raised the (domestic) issue, I was bluntly told on one occasion, 'Why does that matter?' And now we’re seeing why it matters."

The conservative Yoon earned wide praise in Western capitals for his signature policy goal of making South Korea a "global pivotal state" by promoting freedom, human rights and the rule of law. That policy saw Seoul weigh in more publicly on the side of Washington and NATO on hotspots such as the South China Sea, Taiwan, and Ukraine.

Philip Turner, who served as New Zealand's ambassador to Seoul until last year, said Yoon had showed autocratic tendencies particularly since the April parliamentary elections but it was seen as a typical political show of strength.

"Like Koreans themselves, including many of Yoon’s own supporters, I don’t think any diplomats thought that Yoon would go as far as to declare martial law with no basis – an inexcusable and inexplicable action from an ex-prosecutor who positioned himself as a champion of democracy," Turner said.

Yoon’s Global Pivotal State approach will not be mourned, given the hypocrisy with which it finally collapsed, but if Yoon is ousted by the Constitutional Court following his impeachment on Saturday, the West will hope the next president upholds much of its substance in practice, Turner said.

Photo from Reuters