When invaders get away with crossing into enemy territory and escaping unharmed or causing damage with impunity, the attacked party is in trouble, often on the verge of collapse. However, resistance to invaders creates no-go areas and is a sign of strength, bad cess for an attacker.

The most famous instance of an invasion portending fatal weakness in an adversary was actually unsuccessful. It is called anabasis, from the description by Xenophon of the Greek army that lost a battle against the Persians in 401 but managed to escape through Anatolia to the Black sea and back to Greece. Seventy years later Alexander the Great crippled the Persian empire in Gaugamela.

The first inkling Rome had that it had reached a limit to its conquests after its subjection of Gaul was when legions led by Varus incautiously entered the Teutoburg forest in Germany and the warlord Arminius destroyed them. Emperor Augustus cried in fury: "Varus, give back my legions!" Rome still had to reach its apogee. It only began declining after 200. The Visigoths, the Sueves, and the Vandals rode through Roman Gaul practically unopposed. In the early 5th century, the Visigoths set up an independent kingdom with its capital in Toulouse. In 475, it was endowed with a constitution by king Euric. The following year Romulus Augustulus, the last Roman emperor (his name suggests either a joke or a child), was deposed and reputedly confined to a villa overlooking the bay of Naples.

In the 8th century, the Chinese Tang empire tried to cut through Turkish territories in the steppes and in Transoxiana (mainly modern Uzbekistan). In Samarkand, the Chinese had a rebellion in their hands. In 751, an alliance of Turks and Arabs put an end to Chinese expansionism in Central Asia in the battle of Talas (today in Kyrgyzstan). The butter for the Chinese got too hard for easy cutting.

By contrast, the Xixia were fearsome nomadic Tibetans who roamed northwestern China at will. In the late 9th century, they founded a kingdom in the Ordos and constrained the Tang dynasty to keep to a heavily fortified corridor to the Tarim Basin, the essential link in the Silk Road, China's most successful international commercial enterprise before our times. The energies spent in defending its western marches plus internal strife led to the downfall of the Tang. The Xixia became vassals of Jenghiz Khan. However, when Jenghiz needed levies for his attack on Khwarizm, the Xixia sent word that if he needed troops he didn't deserve to rule. Jenghiz bided his time and in 1226 started an onslaught on Xixia. He died before capturing the Xixia capital, but he left graphic instructions that Xixia bones should be strewn over the landscape. They were.

The Seljuks were a branch of a Turkic mother horde called Oghuz. They went on their own stomping through Transoxiana and conquering Persia and Iraq. Anatolia was part of the Byzantine Empire, but as good nomads the Seljuks were not deterred by borders. They habitually broke into Byzantine lands. In 1071, Alp Arslan, the Seljuk sultan of Persia, was foraying through eastern Anatolia. Emperor Romanos IV Diogenes was leading an army to chastise the Turks. The two forces met al Manzikert (Turkish Malazgirt) and the Seljuks crushed the Greeks. The Byzantine Empire had still various centuries of life in it, but after this encounter it virtually lost Anatolia to the Turks. Without Anatolia, and later without the Balkans, it was a matter of time before the Turks gave it its quietus.

Russia was originally Kievan Rus, a congeries of principalities from Novgorod to Kiev and east to the Urals. After blandishing their horsehair emblems all over the Central Asian steppes, the Mongols crossed southern Russia and the Ukraine and raided central Europe. They burned Krakow. In Silesia they were met by an army of Germans and Poles, which they routed at Liegnitz (1241). It is often said that the Mongols abandoned their European campaign when they learned of the great khan Ogodei's death. But even before the news reached them, they did not pursue their advance into Germany but wheeled south and raided the Balkans, which probably means they knew they were on a slippery slope.

In Russia, the Mongols came and went and in 1252 they forced Alexander Nevsky, prince of Vladimir, in the heart of Russia, to render them homage. The Mongols, better known as Tatars by the Russians, were overlords of Russia until 1480, when they retreated before a Russian army at the river Ugra.

When centuries later, Charles XII of Sweden tried to do to Russia what the Mongols had done, Peter the Great let him through inland to the Ukraine, where his forces overwhelmed the Swedes at Poltava (1709). In hot pursuit, Peter entered Ottoman territory and he in turn was cornered by the Ottomans in the river Pruth and had to relinquish lands he had previously conquered.

The knife-through-butter test was applied by Cortez and Pizarro in México and Perú respectively. Armed with stone age weapons, Aztecs and Incas were quickly subdued. The British applied the test at Baksar (1764), in Bihar, and found, as expected, that the Mughal empire had lost all imperial density. The American Civil War was effectively over once Gen. William T. Sherman invaded the heart of the Confederacy from Chattanooga and marched almost unimpeded to the sea and up and into North Carolina.