Hmm...
Well clearly Wales has to avoid English conquest. However, the long term prospects for Wales are bleak unless some kind of permanent understanding is reached with England - perhaps a form of vassalage remaining under a Welsh Prince.
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During the Age of Exploration England settles colonies in North America, resembling mostly OTL USA. However, Wales also gets in on the action, founding some small colonies in OTL New England called New West Wales (a play on Australia's New South Wales). New West Wales is a profitable colony, albeit it is dwarfed by the larger English colonies to the south and its protection is enforced, in effect, by the English Navy.
After a devastating colonial war between France and England over mastery of Canada, New West Wales serves as an important staging ground for English incursions into French Canada. The end of the war sees England expel French influence from the continent, but the war has been costly for England and its loyal suppliant Wales. Tensions in the English colonies lead to a protracted series of civil disturbances which eventually culminate into a full-blown revolution. During this time, and owing to Wales' small colonial empire, the Welsh Prince pays close attention to affairs in New West Wales and grants a number of significant concessions which results in the colonial subjects being, for the most part, a happy bunch.
Nevertheless Wales is expected to support its benefactor England in the American Revolution and it duly provides material support and military support. While many New West Welshmen hold some sympathies with the American rebels, many also serve alongside English regiments in attempting to suppress the American revolt. During a battle outside of New York with American rebels a Welsh regiment is forced to retreat and the commanding officer decides to burn him supplies and that of the local villages to deprive the Americans of supplies. This affair earns significant condemnation in rebel propaganda, and a famous ballad composed by rebel soldier John 'Jack' Wilson sings of the "War against mother England and her bastard son Wales!"
After several years of protracted warfare, and the intervention of various continental empires, England and the United States negotiate a peace settlement. The independence of the United States is recognised, with England retaining superior trading rights and taking possession of territories to the West. An aggressive American negotiating position disputing the boundary of New West Wales is resisted by England despite the territory in question having originally formed part of the English colonies. Nonetheless the territory was never held by rebel soldiers during the war, and the Americans do not press their claim.
Peaceful relations are restored in the Western Hemisphere and trade is prosperous between the three nations England, Wales and USA. During this time further political concessions are made to New West Wales improving its autonomy, albeit it still forms an integral part of the Welsh state. Relations are generally peaceful with the USA. There is a small minority of New West Welshmen who sympathise with America and desire independence for their own state, this is exacerbated by many "loyalists" who fled from America following the end of the Revolution.
Despite the friendly relations tensions flare again within a generation with the continent of Europe embroiled in a large scale war. England, Wales and a vassalised Scotland fight alone against a resurgent French Republic under the leadership of a militarily capable Senate. England faces a desperate battle against the combined navies of France, Spain and Denmark - which leads them into aggressive press ganging of foreign sailors, notably American sailors. With this outrage against American pride, together with England's isolation in the continental war, the USA uses the opportunity to declare war against Wales and reclaim the disputed territory.
An initial victory for the American army is overshadowed when the English navy brings its formidable fleet to bear along the American coast and shells many cities. A punitive raid against the American capital of Philadelphia is repulsed, which restores American spirits. Nonetheless the war bogs down into stalemate as newly arrived regiments from Wales bolster the defence of New West Wales. With the English Navy continuing to assail the American coast, French reversals on the continent, and a threatened English invasion from Canada, America negotiates a peaceful settlement with England and Wales effectively restoring the status quo. Despite the ambivalent outcome, the war is seen as an important national moment for both America and New West Wales.
The three countries never go to war again, and the disputed territory is eventually settled in 1875 by a joint-judicial review with judges from USA, Canada and New West Wales effectively ruling on a compromise.