A defense line or fortification line is a geographically recognizable line of troops and armament, fortified and set up to protect a high-value location or defend territory, which is an "area of defense".
A defense line may be based on natural difficult terrain features, such as rivers or marshes, mountain ranges, or coastlines; temporary field fortification works such as trenches; and/or more permanent fortifications such as fortresses and bunkers.
The invasion of Poland, also known as the September Campaign, Polish Campaign, and Polish Defensive War of 1939 (1 September – 6 October 1939), was a joint attack on the Republic of Poland by Nazi Germany, the Slovak Republic, and the Soviet Union, which marked the beginning of World War II. The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact. The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty.
The aim of the invasion was to disestablish Poland as a sovereign country, with its citizens destined for extermination. German and Slovak forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Germany–Poland border to more established defense lines to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. On 3 September, based on their alliance agreements with Poland, the United Kingdom and France declared war on Germany; in the end their aid to Poland was very limited. France invaded a small part of Germany in the Saar Offensive, and the Polish army was effectively defeated even before the British Expeditionary Force could be transported to continental Europe.
A charge is an offensive maneuver in battle in which combatants advance towards their enemy at their best speed in an attempt to engage in a decisive close combat. The charge is the dominant shock attack and has been the key tactic and decisive moment of many battles throughout history. Modern charges usually involve small groups of fireteams equipped with weapons with a high rate of fire and striking against individual defensive positions (such as a concertainer or bunker), instead of large groups of combatants charging another group or a fortified line.
A breakthrough occurs when an offensive force has broken or penetrated an opponent's defensive line, and rapidly exploits the gap.
Usually, large force is employed on a relatively small portion of the front to achieve this. While the line may have held for a long while prior to the breakthrough, the breakthrough happens suddenly when the pressure on the defender causes him to "snap".
The Hindenburg Line (Siegfriedstellung, Siegfried Position) was a German defensive fortification built during the winter of 1916–1917 on the Western Front in France during the First World War. The line ran from Arras to Laffaux, near Soissons, on the Aisne. In 1916, the Battle of Verdun and the Battle of the Somme left the German western armies (Westheer) exhausted and on the Eastern Front, the Brusilov offensive had inflicted huge losses on the Austro-Hungarian armies and forced the Germans to take over more of the front. The declaration of war by Romania had placed additional strain on the Imperial German Army and the war economy.
The Siegfriedstellung (Hindenburg Line) built behind the Noyon Salient, was to replace the old front line as a precaution against a resumption of the Battle of the Somme in 1917. By devastating the intervening ground, the Germans could delay an Anglo-French offensive. A shortened front could be held with fewer troops and with tactical dispersal, reverse-slope positions, defence in depth and camouflage, German infantry could be conserved. Unrestricted submarine warfare and strategic bombing would weaken the Anglo-French as the German armies in the west (Westheer) recuperated. On 25 January 1917, the Germans had 133 divisions on the Western Front but this was insufficient to contemplate an offensive.