The pilot in the the warbird airplane most often uses the compass to find and fly the correct heading to target. Such a compass usually aligns itself with the Earth's magnetic field outside the airplane. As you can easily see in the world maps below, the lines of the earth's magnetic field do not coincide with the true meridians spanning the globe between the North and South poles but after originating from the magnetic North and South poles they criss-cross all over the place and differ quite a bit depending on continent and region. It is these magnetic field lines that the compass needle aligns itself with and when the compass shows 0° (magnetic due North), the airplane flies parallel to the magnetic field of the earth in this place and time. Different compass headings show the plane's heading relative to this magnetic field.
When navigating, be it in planning or during the flight, a pilot must relate this magnetic heading (relative to the magnetic field) to the true heading (relative to the meridians pointing to the earth's geographic North pole, = true North, = geographic North). The difference between both is called the magnetic variation. If the magnetic field deviates to the East from the true meridian, then it is given in positive numbers and labeled "East", e,g. over Papua New Guinea in 1942 in the map below as magVar = +5°E. A magnetic field variation to the West of the true meridian is labeled with negative numbers and "West", e.g. over Island in 1942 as magVar = -30°W. In places where the map says "No Variation" the direction of the magnetic field lines coincide with the true meridians and the pilot can trust that his compass shows the same numbers as the true heading (magHDG = trueHDG).
When planning a warbird flight on your virtual navigation desk, the magnetic heading (= compass heading) must be determined from the true heading to target as measured with the navigation plotter on the map depending on the current magnetic variation in this region and year. In order to convert true heading to magnetic heading, any positive (Eastern) magnetic variation must be subtracted from the true heading ("East is least"), whereas any negative (Western) variation must be added to the true heading ("West is best").
Mentioning the year is important because the magnetic field and with it the magnetic field lines move with the changing of the Earth's field and moving magnetic North and South poles (see map and caption below).
