China's investment in North Korea worked quite well. Failed in Vietnam but also made sure everyone else failed and now Vietnam government makes a pilgrimage to Beijing every time they change leadership.
Let's also look at Syria.
How much did the US spend? $1 billion annually.
Russia spent $700 million per campaign in comparison.
Investing in culturally similar countries is a winning strategy. US success in South Korea and Japan was only possible due to the post-WW2 moment. If they didn't intervene in the Korean War, there would only be DPRK and Japan wouldn't have made more blood money off of Koreans, making Japan into a poorer frontline state that would've still had significant socialist influence and instability.