Luxembourg after 1815 was not meant to be part of the United NL, but it was in personal union with the King (who was the Duke of Lux). No Dutch rule applied in Lux and the city/fortress received a Prussian garrison (including on paper 1/4 Dutch troops).
Apart from the personal interests of the Dutch King, the United NL would not be interested in territorial expansion, just trade and colonies- for which you need international peace. There were plenty of Dutch politicians IRL who wanted to lose Lux, because being involved their through the King constituted a security risk and the country could be drawn in a Franco-Prussian war.
This almost happened in 1867 when France almost bought Lux from the Dutch King for 5 million guilders, a sale that was vetoed by Prussia (it was a casus belli). Nevertheless, the Prussian fortress was dismantled. Lux was guaranteed eternal neutrality at the Treaty of London of that year (a cause mediated by Russia
). A few years later it would still come to war, though Lux stayed out of it. The personal union of the Dutch Royal house with Luxembourg came to an end in 1890 already, because the male line died out and Lux found another family to run as dukes.I think a United NL would have been better off without Lux (as it was on the map shown above). So they would have let it go in our scenario and may have even added a few adjoining lands (Bouillon, Eastern Liege prov) to go with it. By now the United NL would run less of a security risk in case of a Franco-Prussian conflict. It would stay behind its Meuse fortresses, while war could rage in the Ardennes. It would have better chances of survival by making itself a bit smaller
No need for anybody to invade. And with the added bonuses of a more homogeneous nation (90% NL speaking), good borders and a clear focus on intl trade and empire. It's possible that Lux falls to Prussia in or after 1870, but that's fine. The German Empire is still the United NL largest trade partner, so let them.Strategic industries like armaments and steel however would have been moved away from the border lands towards safer Antwerp or Dutch cities to the north (this was Dutch policy before WW1). That means Walloon cities do not grow as much as IRL and do not receive Flemish workers that will become French speaking over the years. Instead Antwerp, Rotterdam and Amsterdam will see a large influx of Walloons who become Dutch speaking over the years. Brussels with its growing number of Dutch speaking bureaucrats and businessmen, without French immigration, will also become a largely Dutch speaking city.
The Army by 1914 can raise about 500.000 with a deeper reservoir of another 500.000. 200.000 are in the Field Army, with 8 divisions and two cavalry Divisions, 200.000 are fortress troops (mainly reserves) and another 100.000 are in the Burgerwacht/
Schutterij. Colonial troops are the 30.000 strong Colonial Army (80% locals) and the 20.000 strong Korps Fusiliers (open to all Europeans as IRL).
Empire:
Like I wrote before only as a neutral small country could the NL survive as a colonial power. In the case of a United NL the Indies are united a bit faster and become a bit bigger. Sumatra is acquired as IRL after an earlier treaty of London (that left Singapore and Malacca in British hands, but did not take care of Borneo). Northern Borneo can thus also be slowly acquired by the Dutch, before adventures such as Brooke can settle in. Maybe the Dutch also acquire eastern parts of New Guinea, instead of the Germans. The extra manpower from the southern NL as well as southern businesses give the Dutch extra momentum to expand its empire as well as raise the number of troops and other Europeans in the colonies to almost twice the number than IRL (from 200000 in 1914 to 350000).
Having lost the Cape to the British, the Dutch establish a supply base on the route towards the Indies at Walvisbaai in the first half of the nineteenth century. They had been there before and it was up for grabs until the 1870s. They do not take all of present day Namibia, but just go south towards the Orange and establish contact with the Boers in other parts of Southern Africa. But the Brits still establish their overland route through Kimberley north and the Dutch will not risk war with them.
So my bet for the survival of the United NL is a smaller European territory - minus Lux, Ardennes, and a larger Indies (plus Sabah/Serawak/eastern New Guinea) and a base at Walvisbaai. That's how it is by 1914. Does the United NL stay out of the war?
I'd say it has a very good chance.
It will have mobilized 500.000 men in August, sitting behind its relatively strong lines of fortresses. It has access to raw materials for armaments in the Indies, it has a modest though well equipped navy and colonial force that is capable of actions far away (and harass German colonies). And it may join the other side if attacked.
Yes, it can be overrun in the south perhaps, but a great cost...