It's a bit of a long one, so strap in.

You're absolutely correct in your assessment, I'll be more than happy to expand on it. We can start very briefly with WW1 and work our way back. After all, Great Britain stood by looking during the Franco-Prussian War, and then joined France in WW1, this might seem puzzling.

The puzzle becomes considerably less mysterious if one focuses on the security policies of the UK during its history. For Great Britain, its decision then to join the Entente actually aligns perfectly with a policy pursued since the days of Henry VII. In actuality, the irony is that Britain’s Splendid Isolation in the second half of the 19th century was the aberration – the anomaly. British intervention in Continental affairs was actually meant to be the norm.

The lack of British intervention in the Austro-Sardinian War of 1859 was possibly compelled by fears of a French invasion. Less than a decade later, Britain would successfully assert its interests against those of France and Prussia during the 1867 Luxembourg Crisis. The crisis started with France attempting to purchase the Grand Duchy from the Dutch, igniting Prussian agitation at the threat to their rights in the area.

The subsequent conference in London denied both Paris and Berlin control of the region, guaranteeing the perpetuation of the personal union between the Grand Duchy of Luxembourg and the Kingdom of the Netherlands. It also led to the demolishing of the great fortifications of Luxembourg, which had long been important and famous, earning Luxembourg the nickname “Gibraltar of the North”. 

Likewise, the absence of British soldiers from the battlefields of the Franco-Prussian War does not equal to a retreat from the matter: on the contrary, Whitehall took actions to preserve the balance of power on the Continent. In 1870, the British threw their weight into the brewing conflict by demanding separate treaties from France and from Prussia, reaffirming their guarantee of Belgian neutrality as contained in the Treaty of London 1839. This further forced both sides to renounce all claims to the Belgian territory.

Due to these diplomatic successes, Great Britain did not need to intervene militarily nor did it desire to. However, this did not represent disentanglement: London never left the Continental powers to do as they pleased. It made it abundantly clear that if either power violated Belgian neutrality, it would march to war on Belgium’s behalf. The two sides were wise enough to recognize this and not risk British hostility.

A quote that perfectly encapsulates British policy can be found in a letter written in 1749, sent by the Duke of Newcastle to Lord Hardwicke: “France will outdo us at sea when they have nothing to fear by land. I have always maintained that our marine should protect our alliances upon the continent; and thus, by diverting the expense of France, enable us to maintain our superiority at sea.”.

Another excellent quote comes from historian Cyril Falls: “Yet too many British historians and self-styled experts in strategy have, especially when dealing with this war [First World War], made preposterous claims for sea power, claims both contrary to common sense and founded on distortion of history. Britain, they say, should not have created a mass conscript army; she should have stood off, kept the seas, and blockaded the enemy. This, they add, was her traditional policy. It may have been the policy she hankered after in selfish moments, but it had never been pursued for long. Time after time Britain – a country with a small population, and therefore a small army, before the 19th century – had been forced to send land forces to the continent to prevent her allies from collapsing. It was not merely that they might be routed and driven out of the war by sheer force of arms. They might walk out.”

Britain first developed the policy of providing security guarantees to her allies on the Continent in the late 15th century.

Henry VII had committed himself to an anti-French alliance with Spain and Austria at the opening of the Italian Wars in the 1490s. This was the beginning of England’s policy of neutralizing the Low Countries and restraining any prospective hegemon, which initially was France and then Spain, following the disintegration of the French Kingdom in the Wars of Religion.

Queen Mary sent troops to help Philip II at St Quentin, Queen Elizabeth sent strong contingents to help the Dutch in their revolt against Spain, and from William III the British committed increasingly powerful forces to the Continent to fight Louis XIV. This was the great age of Marlborough when Britain contributed to such mighty victories as Steinkirk, Oudenarde, Malplaquet, Ramillies, and Blenheim.

Britain’s access to naval bases the world over allowed an unprecedented ability to project military power. It also gave London an undisputed role steering world trade, something which her European competitors perceived as an undue influence on their ability to obtain economic prosperity. This was acutely felt in Germany, where the great economic and industrial progress of the 19th Century depended primarily on the Reich’s export trade – which in turn depended on Great Britain, as the chief controller of the world’s oceans.

The American political scientist, Nicholas Spykman, observes in "America's Strategy in World Politics": “In times of continental threats to her naval supremacy Great Britain usually deserted her policy of isolation and joined one of the continental combinations. Such a step improved the relative power position, reduced the size of the naval strength to be met, and increased the size of the fleet with which to meet it. A continental ally was helpful in the early stages of a power struggle and absolutely indispensable when war broke out. Only through a land power of the mainland could Britain take a successful offensive against a large continental state. Only with the army of an ally could the land frontier of the opponent be attacked and the type of action developed which, together with the British blockade, might bring victory.”

He also points out that “Great Britain has played an active role in most of the coalitions that have been formed to restrain growing continental powers. In the name of preserving the equilibrium everybody in Europe has fought everybody else. Britain has successively defeated Spanish, Portuguese, Dutch, French, and German sea power and has successfully used Spain, Portugal, Holland, France, and Prussia as allies. Power politics is responsible for her reluctance to make advance commitments to any nation beyond the buffer zone. He who plays the balance of power can have no permanent friends.”