Flanking maneuver
Examples are the Second World War's German Winter Line in Italy anchored by the Tyrrhenian and Adriatic Seas and the trench systems of the Western Front which ran from the North Sea to the Alps. Attacking such positions would be expensive in casualties and more than likely lead to stalemate. Flanking attacks into areas outside the main zone of contention might be attempted to break the stalemates.
If successful, such operations can be shattering, such as at the Inchon, and break into lightly held rear echelons of an enemy when its front line forces are committed elsewhere. Even when they are not entirely successful, such as at Anzio, the operations can relieve pressure on troops on the main battle front by forcing the enemy to divert resources to contain the new front.
The operations may have strategic objectives such as the Invasion of Italy itself, the Gallipoli, and the Normandy landings.
Such a strategy is not new, as Hannibal attacked Rome by going over the Alps, rather than taking the obvious route. In return, Scipio Africanus was able to defeat Hannibal not by trying to face him in Italy but by first undermining his power base in Iberia and then attacking his home city, Carthage.

Strategic flanking: Flank attacks on the strategic level are seen when a nation or group of nations surround and attack an enemy from two or more directions, such as the Allies surrounding Nazi Germany in World War II.  In those cases, the flanked country usually has to fight on two fronts at once, placing it at a disadvantage.
The danger of being strategically flanked has driven the political and diplomatic actions of nations even in peacetime. For example, the fear of being strategically flanked by the other in The Great Game between the British and the Russian Empires led to the expansion of both of them into China and the British eastwards into South-East Asia. The British feared that British India would be surrounded by a Persian and Central Asia satellite to Russia in the west and north and a Russian-dominated China in the east. A China under British influence would mean that the Russians would be penned in from the south and the east. Initially, the Russians were more successful than the British in gaining territorial concessions in China, but the British were able to counteract that by cultivating the emerging Empire of Japan as a counterweight to the Russians, a relationship that culminated in the Anglo-Japanese Alliance.
The Cold War had its own Great Game, with the US and USSR competing for influence in Europe, Asia, Africa and Latin America. The division of Europe, the proxy wars in Asia, and events like the Cuban Missile Crisis were all of great strategic importance to the two competing hegemonies.

Historical examples: 

See also: Battleplan (documentary TV series)
Pincer movement
Encirclement


== References ==