>I think it's quite telling the author had to go back to the 15th century to find a historical precedent for his claim.
To me, it is just a period with an anecdote the author was familiar with. The War of the Roses, while an important event in British history, isn't the type of nation destabilising event you're looking for either. It is yet another succession crisis that happened in monarchies time-to-time that was militarised. The argument being that the militarisation is the result of failed foreign military campaigns.
>Why not take an example from the end of the British Empire, the French Empire or the Japanese Empire
All fell more or less as a protracted result of the Second World War followed by the Cold War. I don't think that is comparable. The Vietnam War and the cultural effect it had on the US probably more apt. That is probably where this first began, and then continued with Afghanistan and Iraq.
>Spain, Portugal and Netherlands
The history of both Portugal and the Netherlands are quite intertwined with Spain and its (ie. the Hasburgs') imperial ambitions. Take the War of Spanish Succession for example, the Spanish ceded what was left of the Spanish Netherlands (after the Dutch revolt) to Austria, and arguably had much more impact than the War of the Roses.
It's a widely accepted view that the imperial activities of these powers in foreign lands (both in Europe and beyond) aggravated the conflicts in Europe over several centuries, culminating in the World Wars.