Invasion! was a series created by Pat Mills and mostly written by Gerry Finley-Day that appeared in the first 51 editions of the weekly comic 2000 AD.

The series introduced the character of Bill Savage, a classic anti-authoritarian character of the type which Mills is best known for. A prequel, Disaster 1990, was published in 1979. The series continued in sequels under the title Savage, beginning in 2004, by Pat Mills and Charlie Adlard.

In the alternate recent history established in Savage, the Soviet Union fell to a fascist military coup and was renamed the Volgan Republic by Marshal Vashkov in 1991. In 1992, vast new reserves of North Sea oil were discovered off the coast of Britain. Ken Livingstone became head of the Labour Party, and his "True Labour" movement gained power over Shirley Brown's Conservative Party in a 1995 general election. In 1996, Prime Minister Livingstone ordered the removal of all American military bases from the British Isles; by 1998, a newly isolationist United States had withdrawn completely from Europe and the North Atlantic Treaty Organization (NATO).

By 1999, Prince Charles was King of the United Kingdom. The United States and Volgan Republic had a secret agreement, carving up the world into spheres of influence. Meanwhile, a fifth column among the establishment, feeling threatened by the new left-wing government, made overtures to the Volgans, offering to help with any invasion.

Set in the year 1999, the first part of the series depicts Western Europe and the United Kingdom's being swiftly overrun by an invasion from the Volgan Republic known as the Eight-Hour War. North Sea oil rigs are seized by helicopter-borne commando units; Heathrow airport is captured by paratroopers, allowing heavy transport planes to land delivering heavy armour close to London; and the Midlands are hit by nuclear weapons, including a 50 megaton warhead. British royal family escape to Canada, The UK Government capitulates, with many hard line anti-Volgan MPs, Lords, and generals along with Prime Minister Shirley Brown being executed; a puppet government, under Prime Minister Simon Creepton, takes control for the puppet state of The People's Republic of Britain.

Bill Savage, an East-End lorry driver, begins a one-man resistance movement against the Volgans after his wife and children are killed when Savage's home is hit by a Volgan tank shell during the invasion. As with similar characters that Mills and Day had created for other titles (such as Action and Battle Picture Weekly), Savage is given a gimmick, his preferred weapons: a hauling hook and a sawn-off shotgun. Initially solo, he is recruited by Lt. Peter Silk into the British resistance. Initially leading the Mad Dogs cell out of the Isle of Dogs, Savage would later be mobile and take part in resistance action throughout the country. Throughout the series, Savage's brutal violence and working-class common sense lead to victory, whereas military resistance is depicted as being class bound and ineffectual.

At the end of the series, Savage is tasked with taking the heir to the throne, Prince John, to safety in Canada. He smuggled the prince into North America on a US cargo ship; as Volgan agents murdered most of the crew trying to stop him, the series ended with Savage believing the United States would now get involved against the Volgans.

The Volgan War (AKA the Fourth Oil War) became part of a connecting back story for a number of 2000 AD series including ABC Warriors.